usb: gadget: u_audio: Use managed buffer allocation

Clean up the driver with the new managed buffer allocation API.
The hw_params and hw_free callbacks became superfluous and dropped.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20191210141822.18705-2-tiwai@suse.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>