mmc: core: Fix typo at mmc_card_sleep

Fix wrong bus_ops->sleep check.  (This isn't expected to have real-world
consequences, because the mmc core always defines both 'awake' and
'sleep' ops.)

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>