usb: gadget: audio: Fix a missing error return value in audio_bind()

If 'usb_otg_descriptor_alloc()' fails, we must return an error code, not 0.

Fixes: 56023ce0 ("usb: gadget: audio: allocate and init otg descriptor by otg capabilities")
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Felipe Balbi <balbi@kernel.org>