msm: clock: Add is_better_rate() helper function

The is_better_rate() function is meant for use in clk_round_rate()
implementations to check if one of the new possible rates is better than
the current best match for the round rate request.

Change-Id: Ifca1840b15ec6faf6031af35ca9d219f6b32c3bb
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>