usb: gadget: pxa27x: Test for a valid argument pointer

A call usb_put_phy(udc->transceiver) must be tested for a valid pointer.
Use an already existing test for usb_unregister_notifier call.

Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Reported-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Petr Cvek <petr.cvek@tul.cz>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>