MIPS: ralink: add missing clk_set_rate() to clk.c

This function was missing causing make allmod to fail.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/8005/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>