clk: sophgo: Fix a warning about void returns value

clk: sophgo: Fix a warning about void returns value

The cv1800b_clk_setfield function returns void, but was doing "return
writel(...);" and while seemingly having a void function return a void
function is not a warning, when readl is a macro this problem is shown.
Correct the code to instead simply call writel.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Yao Zi <ziyao@disroot.org>