msm: clock: Allow msm_clock_init to be invoked multiple times

In preparation for a series of changes that move MSM clock
controllers to the Linux device model, allow msm_clock_init
to be invoked multiple times (once for each clock controller).

Change-Id: I0d56612c660f020ea762feb468234813cff0db22
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>