USB: serial: fix memleak in driver-registration error path

udriver struct allocated by kzalloc() will not be freed
if usb_register() and next calls fail. This patch fixes this
by adding one more step with kfree(udriver) in error path.

Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>