p54usb: fix leaks at failure path in p54u_probe()

If p54u_load_firmware() fails, p54u_probe() does not deallocate
already allocated resources. The patch adds proper failure handling.

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

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