usb: gadget: u_audio: factorize ssize to alsa fmt conversion

Factorize format related code common to the capture and playback path.

Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20210118084642.322510-3-jbrunet@baylibre.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>