mmc: sdhci: initialize sdhci_host lock in sdhci_alloc_host()

Currently, the sdhci host lock is initialized in sdhci_add_host()
but there can be a case where it is required even before that.
Hence, initialize it in sdhci_alloc_host() where sdhci_host
structure is allocated.

Change-Id: If99d82679c07bc2d36e0aad9354757288aa400b8
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>