USB: serial: garmin_gps: fix memory leak on failed URB submit

Make sure to free the URB transfer buffer in case submission fails (e.g.
due to a disconnect).

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>