mmc: core: dont send PON during suspend

During suspend, eMMC VCC can be turned off. So instead of sending
a power off notification, send a sleep command. According to the
eMMC 4.5 specification, this is the only time where its clearly
mentioned that the VCC regulator can be turned OFF.

This has been clarified in eMMC 5.0 specification and this applies
even if PON with sleep is not sent to the card.

Change-Id: I6c424bed3158132af7f9c2a2a701af7369fd5ec7
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>