mmc: core: Make sure host is active before sending pon command

Add mmc_get_card() to make sure host is resumed before sending pon
command. Also, add mmc_put_card() after completing pon command so
that it can release the host and also vote for runtime suspend.

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Change-Id: I52f4d854388a608c6a09e55a693e3b36bd801fd8