ARM: mach-imx: clk-busy: Staticize clk_busy_mux_ops

Fix the following sparse warning:

arch/arm/mach-imx/clk-busy.c:150:16: warning: symbol 'clk_busy_mux_ops' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>