apple: program the NHI interrupt throttle via ring->interval_nsec

[afrog33k]

What this fixes

On an Apple NHI the interrupt throttle register lives at 0xd004c, not the
stock 0x38c00 that tb_ring_throttling() programs. The kernel's own ring
activation writes the Apple one from ring->interval_nsec, and skips the
write when that field is zero — which it always was, because this module
never set it.

So on Apple hosts the throttle was silently skipped, the register kept its
firmware default, and the link carried a latency floor that did not move with
message size: 2 B through 4 KB all measured ~65.3 us (ib_send_lat across a
USB4 link).

The change

• Set ring->interval_nsec for both rings before tb_ring_start() on Apple
  hosts. This is not gated on the peer's backend: the NHI is Apple's
  regardless of who is on the other end of the cable, so native-backend rings
  toward a Linux peer need it just as much. (tbv_path_is_apple() is
  backend-based and would skip exactly the leg that needs this.)
• Log the before/after value so the write is observable.
• Guard the field access behind a Makefile compile probe
  (TBV_HAVE_RING_INTERVAL_NSEC), so stock kernels — where struct tb_ring
  has no such member — still build.

Measured

Before/after on the same link, same harness (ib_send_lat, 2 B, warm floor):

	typical
before	65.3 us, flat from 2 B to 4 KB
after	14.8 us, scaling with size

The stock-NHI half of the same fix (writing 0x38c00 directly on a host whose
kernel does not export tb_ring_throttling()) is deliberately not in this
PR — it needs a nhi_throttle_direct-style opt-in and its own discussion.

Verified: builds against the Asahi 7.1.13 USB4 tree with no new warnings.

Context: found while bringing up RDMA over a USB4 cable between an Asahi
host and a stock-kernel AMD host. Write-up:
https://boracode.ai/blog/thunderbolt-no-incarnation

[hellasbot]

Hydra reported failures for this PR.

• Eval: #158839
• Jobset: hellas/thunderbolt-ibverbs-pr-75
• Head: 36a47f13bf2b

Failed builds:

• x86_64-linux.checks.script-syntax - aborted (log, tail, raw)
  • thunderbolt-ibverbs-script-syntax-0.3.4 - aborted (tail, raw) - �[31;1merror:�[0m failed to start SSH connection to '�[35;1mstrix-1.hydra-builders�[0m'
  • thunderbolt-ibverbs-script-syntax-0.3.4 - aborted (tail, raw) - �[31;1merror:�[0m the daemon is missing the '�[35;1mrealisation-with-path-not-hash�[0m' protocol...
  • thunderbolt-ibverbs-script-syntax-0.3.4 - aborted (tail, raw) - �[31;1merror:�[0m the daemon is missing the '�[35;1mrealisation-with-path-not-hash�[0m' protocol...
• x86_64-linux.checks.thunderbolt-ibverbs - failed (log, tail, raw)
  • thunderbolt-ibverbs-0.3.4 - failed (tail, raw)