clk: sunxi-ng: nk: Make ccu_nk_find_best static

make C=2 reports:

  CHECK   drivers/clk/sunxi-ng/ccu_nk.c
drivers/clk/sunxi-ng/ccu_nk.c:17:6: warning: symbol 'ccu_nk_find_best' was
not declared. Should it be static?

ccu_nk_find_best is only used within ccu_nk.c. So make it static to get
rid of this warning.

Fixes: adbfb005 ("clk: sunxi-ng: Add N-K-factor clock support")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>