msm: clock-copper: Switch max_div values from 16 to 15.

The dividers on branches (cdivs) can be configured using
4 bits in the CBCR register. The range of possible values
is therefore 0 through 15. Fix max_div to reflect this
maximum value.

Change-Id: I11325eb371d93254b5327b1ded56b1820661713f
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>