msm: msm_bus: Only set clock rate on unset clocks.

Checks to see if the clock is already set before
requesting a rounded(1) rate.

CRs-Fixed: 568026
Change-Id: I7adbcffd7c86c4e714573aad5bd3813480db2a48
Signed-off-by: Dan Sneddon <dsneddon@codeaurora.org>