mmc: mmc_card_keep_power cleanups

mmc_card_is_powered_resumed is a mouthful; instead, simply use
mmc_card_keep_power, which also better explains the purpose of
the macro.

Employ mmc_card_keep_power() where possible.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Chris Ball <cjb@laptop.org>