serial: ar933x: Remove unneeded static structure

In case that no RTS GPIO is available do not use a dedicated nullified
serial_rs485 struct to disable RS485 support, but simply delete the
SER_RS485_ENABLED flag in the ports rs485_supported struct.
This make the structure superfluous and it can be removed.

Signed-off-by: Lino Sanfilippo <l.sanfilippo@kunbus.com>
Link: https://lore.kernel.org/r/20240407002709.16224-5-l.sanfilippo@kunbus.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>