mmc: core: fix race between mmc_power_off and mmc_power_up

In case card detect IRQ is triggered before mmc_add_host(), then
there could be a potential race between mmc_power_off() which gets
called from mmc_rescan() of card detect IRQ handler and
mmc_start_host() which gets called from mmc_add_host(). This may
turn off the clocks while mmc_start_host() is still running and thus
may result in an un-clocked register access.

Change-Id: I522df75ba0ad23f065127e015ad825075be94596
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>