usb: gadget: u_audio: Add capture/playback srate getter

UAC1/UAC2 functions will need to query u_audio about the currently set
srate. Add the getter functions.

Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20220121155308.48794-5-pavel.hofman@ivitera.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>