serial: omap: fix rs485 probe on defered pinctrl

If the gpio is not yet available we better also
defer the probing in the rs485 case.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>