msm_serial: Convert to clk_prepare/unprepare

Add calls to clk_prepare and unprepare so that MSM can migrate to
the common clock framework.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David Brown <davidb@codeaurora.org>