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: I67a025606b03bb3cc21e818c2b4b519aa2b53888
Signed-off-by: Sarthak Garg <sartgarg@codeaurora.org>