mmc: core: remove redundant card null check to mmc_can_sleep()

Note that only _mmc_suspend() will call mmc_can_sleep(). And card is
checked before in mmc_can_poweroff_notify().

Signed-off-by: Yue Hu <huyue2@yulong.com>
Link: https://lore.kernel.org/r/20210115034506.646-1-zbestahu@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>