mmc: omap: Fix possible NULL pointer deref

Either OMAP_MMC_STAT_CARD_ERR or OMAP_MMC_STAT_END_OF_CMD might fire
if there is no host->cmd pointer.

Check for a valid host->cmd pointer before calling mmc_omap_cmd_done().

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Chris Ball <cjb@laptop.org>