mmc: sdhci: remove incorrect spinlock release

sdhci_set_clock() may try to release spinlock even if it is not acquired.
Fix this by removing spin lock/unlock before usleep() as sdhci_set_clock()
is not getting called with spinlock held.

Change-Id: I4db5e818daf8c9e0f29b094120f0df3aa3eb073e
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>