clk: k210: Fix k210_clk_set_parent()

clk: k210: Fix k210_clk_set_parent()

In k210_clk_set_parent(), add missing writel() call to update the mux
register of a clock to change its parent. This also fixes a compilation
warning with clang when compiling with W=1.

Fixes: c6ca7616f7d5 ("clk: Add RISC-V Canaan Kendryte K210 clock driver")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Link: https://lore.kernel.org/r/20210622064502.14841-1-damien.lemoal@wdc.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>