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

commit c4ac4496e835b78a45dfbf74f6173932217e4116 upstream.

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