MMC: OMAP: Check the get_cover_state function pointer if not set

If the get_cover_state is not set, it occurs the oops.

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>