usb: musb: convert musb to new style bind

udc-core checks for valid callbacks so there is no need for the driver
to do so. Also "can-be-bound-once" is verified by udc-core. The pull-up
callback is called by udc-core afterwords.

[ balbi@ti.com : keep holding gadget_driver pointer for now
		 remove the stupid check for gadget_driver otherwise
		  we don't handle IRQs ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>