serial: 8250: omap: Use serial8250_em485_supported

8250_omap uses em485, fill in rs485_supported accordingly. This makes
RS485 work with 8250_omap again, which was broken with the introduction
of the RS485 config sanitization.

Fixes: be2e2cb1d2819 ("serial: Sanitize rs485_struct")
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Link: https://lore.kernel.org/r/20220916110955.161099-1-matthias.schiffer@ew.tq-group.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>