mmc: core: Avoid mmc_power_off during shutdown/reboot

During shutdown/sleep putting vdd-io in LPM mode after sending the
Poweroff notify command is leading to OCP.

Avoid putting vdd-io in LPM mode during shutdown/sleep.

This change is inline with earlier kernels.

Change-Id: I08698bc8b4f1c420910747b6374485f6265d553b
Signed-off-by: Sarthak Garg <sartgarg@codeaurora.org>