clk: qcom: clear div mask before assigning a new divider

The current behaviour does a bitwise OR of the previous and new
divider values, this is wrong as some bits may be set already. We
need to clear all the divider bits before applying new ones.

This fixes potential issue with 1Gbit ethernet on SA8155P-ADP boards.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
[caleb: minor wording fix]
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>