mmc: core: add power on to SD card detect

In commit aaf56cab2137 ("mmc: core: power off host if
SD card resume fails"), the mmc host is powered off if
SD card init failed during resume. However, the power
is not turned on before next time SD card detect.
It would not cause crash or malfunction but lead to
too many warnings. This change adds power on to
function mmc_sd_detect to make sure the power is on
before send commands to SD card.

Change-Id: I7c85ec4d6b561d167f9ff77a71f8e00f5860e8d2
Signed-off-by: Can Guo <cang@codeaurora.org>