serial: ar933x: Fix check for RS485 support

RS485 is not possible without an RTS GPIO regardless of whether RS485 is
enabled at boot time or not. So correct the concerning check in the probe()
function.

Fixes: e849145e

 ("serial: ar933x: Fill in rs485_supported")
Signed-off-by: Lino Sanfilippo <l.sanfilippo@kunbus.com>
Link: https://lore.kernel.org/r/20220710164442.2958979-2-LinoSanfilippo@gmx.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>