mmc: sdhci: Fix unclocked register access

During platform driver probe we call mmc_start_host in sdhci_add_host,
which could start the mmc_rescan work immediately and trigger a runtime
suspend. This creates a race condition where the clocks could be turned off
even before the probe has completed leading to unclocked register access.

CRs-Fixed: 770843
Change-Id: I77ae36f805e496d56ed96db3ccaa83f2c37c926c
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>