mmc: sdhci-msm: skip eMMC slot probe if eMMC isn't a bootdevice

If eMMC is not a primary bootdevice, there isn't any point of probing
eMMC device hence disable the probing in such case.

Change-Id: I92fa8c2ef373fd8a9140dbfb41356684aaa28e4e
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>