USB: serial: drop bogus to_usb_serial_port() checks

The to_usb_serial_port() macro is implemented using container_of() so
there's no need to check for NULL.

Note that neither bus match() or probe() is ever called with a NULL
struct device pointer so the checks weren't just misplaced.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Johan Hovold <johan@kernel.org>