usb: gadget: function: Added usb_assign_descriptors failure check

Added failure check for usb_assign_descriptors call in bind function.

Acked-By: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Pavitrakumar Managutte <pavitra1729@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>