audio_device: enforce the documented FIFO minimum in the EP-IN flow-control guard

The comment above audiod_tx_packet_size() states flow control needs a FIFO
of at least 4*Navg, but the guard tests nominal_size[1] <= fifo_depth * 4 -
true for any FIFO larger than a quarter packet - instead of
nominal_size[1] * 4 <= fifo_depth. As written, flow control engages on
FIFOs far below its own documented minimum, where the depth/2 setpoint sits
within one packet of empty and the packet_size = 0 branch (a zero-length
packet, i.e. an audible 1 ms dropout for audio-class hosts) is reachable
from ordinary scheduling jitter rather than only from gross clock
deviation. With the guard corrected, undersized FIFOs fall back to the
plain min(count, max) path as intended.

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