usb: gadget: f_uac2: Fix pcm sample size selection

The pcm playback and capture sample size format was fixed
SNDRV_PCM_FMTBIT_S16_LE.

This patch respects also 16, 24 and 32 bit p_ssize and c_ssize values.

Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Sebastian Reimers <sebastian.reimers@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>