mmc: omap_hsmmc: fix NULL pointer dereference for dt boot

dev->platform_data is NULL in case of device tree boot,
instead use the saved version in struct omap_hsmmc_host.

Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>