usb: gadget: add platform module alias where it is missing

Without it udev won't be able to load the driver once it notices the
device unbound.

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>