serial: 8250_port: unify check of em485 variable

Unify the check of em485 variable to be either (em485) or (!em485) instead of
the explicit comparison to NULL.

While here, remove redundant check in __do_stop_tx_rs485() and
__stop_tx_rs485() since the functions ain't called with NULL value of em485
variable.

Cc: "Matwey V. Kornilov" <matwey@sai.msu.ru>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>