msm: clock-generic: Update the rate field on mux parent change.

Changing the parent of a mux changes its rate to the rate of the new
parent.  So, update the rate field of the clock when changing the parent of
the clock.

Change-Id: Ib219a5eb025c5bde922eed1864ce5296c7e84a8d
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>