mmc: omap_hsmmc: support deferred probing for GPIOs

If the CD/WP-GPIOs are not provided by the SoC's GPIO controller,
we need to handle the case where omap_hsmmc is probed earlier than
the GPIO controller chosen in the device tree.

Fix this by checking the return value of of_get_named_gpio against
-EPROBE_DEFER and passing it through to the probe function.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Acked-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>