clk: socfpga: stratix10: Optimize local variables

Since readl() returns a u32, the local variable reg can also have the
data type u32. Furthermore, mdiv and refdiv are derived from reg and can
also be a u32.

Since do_div() casts the divisor to u32 anyway, changing the data type
of refdiv to u32 removes the following Coccinelle/coccicheck warning
reported by do_div.cocci:

  WARNING: do_div() does a 64-by-32 division, please consider using div64_ul instead

Compile-tested only.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>