Avoid possible crash if CONFIG_MMC_OMAP_HS is not set

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Madhusudhan Chikkature <madhu.cr@ti.com>
Cc: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Kishore Kadiyala <kishore.kadiyala@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>