USB: garmin_gps: remove bogus disconnect test in bulk callback

Remove bogus disconnect test for serial device being NULL in read bulk
callback. This can never happen as the port read urb is killed (and
poisoned) at close, which in turn 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>