clk: u300: Silence sparse warnings

drivers/clk/clk-u300.c:1175:13: warning: symbol 'u300_clk_init' was not declared. Should it be static?

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>