USB: serial: garmin_gps: fix memory leak on probe errors

commit 74d471b598444b7f2d964930f7234779c80960a0 upstream.

Make sure to free the port private data before returning after a failed
probe attempt.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>