clk: divider: Fix overflow in clk_divider_bestdiv

Commit c686078 ("clk: divider: Add round to closest divider") introduced
a helper function to check whether given divisor is the best one instead
of direct check. However due to int type used instead of unsigned long
for passing calculated rates to this function in certain cases an
overflow could occur, for example when trying to obtain maximum possible
clock rate by calling clk_round_rate(..., UINT_MAX).

This patch fixes this issue by changing the type of rate, now and best
arguments of the function to unsigned long, which is the type that
should be used for clock rates.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>