serial: 8250: Remove superfluous sanity check

The serial core already checks the RS485 RTS settings for sanity, so remove
the superfluous check in serial8250_em485_config().

Signed-off-by: Lino Sanfilippo <l.sanfilippo@kunbus.com>
Link: https://lore.kernel.org/r/20240407002709.16224-4-l.sanfilippo@kunbus.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>