clk: mediatek: fixed static checker warning in clk_cpumux_get_parent call

Fixed the signedness bug returning '(-22)' on the return type as u8 with
removing the sanity checker in clk_cpumux_get_parent() since
clk_cpumux_set_parent() always ensures validity in clk_cpumux_get_parent()
got called.

Fixes: 1e17de90 ("clk: mediatek: add missing cpu mux causing Mediatek cpufreq can't work")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>