USB: garmin_gps: remove bogus disconnect test in close

Remove bogus disconnect test for serial device being NULL in close. This
can never happen as close is guaranteed to be called before the last tty
reference is dropped (and port->serial is cleared).

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>