greybus: gb-beagleplay: fix sleep in atomic context in hdlc_tx_frames()

Summary

In the Linux kernel, the following vulnerability has been resolved: greybus: gb-beagleplay: fix sleep in atomic context in hdlc_tx_frames() hdlc_append() calls usleep_range() to wait for circular buffer space, but it is called with tx_producer_lock (a spinlock) held via hdlc_tx_frames() -> hdlc_append_tx_frame()/hdlc_append_tx_u8()/etc. Sleeping while holding a spinlock is illegal and can trigger "BUG: scheduling while atomic". Fix this by moving the buffer-space wait out of hdlc_append() and into hdlc_tx_frames(), before the spinlock is acquired. The new flow: 1. Pre-calculate the worst-case encoded frame length. 2. Wait (with sleep) outside the lock until enough space is available, kicking the TX consumer work to drain the buffer. 3. Acquire the spinlock, re-verify space, and write the entire frame atomically. This ensures that sleeping only happens without any lock held, and that frames are either fully enqueued or not written at all. This bug is found by CodeQL static analysis tool (interprocedural sleep-in-atomic query) and my code review.