usb: gadget: u_audio: Drop superfluous ioctl PCM ops

PCM core deals the empty ioctl field now as default.
Let's kill the redundant lines.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20191210141822.18705-3-tiwai@suse.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>