clk: optimize the divider walk in clk_divider_bestdiv()

Because _next_div() returns a valid divider, there is no need to
consult _is_valid_div() for the validity of the divider in every
iteration.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>