p54usb: fix leak at failure path in p54u_load_firmware()

If request_firmware_nowait() fails in p54u_load_firmware(),
p54u_load_firmware_cb is not called and no one decrements usb_dev refcnt.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: John W. Linville <linville@tuxdriver.com>