snd_uaudio(4): Adapt buffer length to buffer_ms tunable.

Adapt the length of the driver side audio buffer to the USB transfer
interval, which is adjustable through the buffer_ms tunable. This
eliminates unnecessary latency in USB audio playback.

To reduce power consumption caused by frequent CPU wakeups, increase the
default buffer_ms value to 4ms. In combination with adaptive buffer
length, this still results in less roundtrip latency compared to the
previous 2ms default.

Extend the buffer_ms value range to 1ms for low latency applications.

MFC after: 2 weeks
Reviewed by: christos
Differential Revision: https://reviews.freebsd.org/D41942