imx/clk-pllv2: fix wrong do_div() usage

do_div() is meant to be used with an unsigned dividend.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>