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

commit 647024a7df36014bbc4479d92d88e6b77c0afcf6 upstream.

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>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>