drivers: clk: st: Fix mux bit-setting for Cortex A9 clocks

This patch fixes the mux bit-setting for ClockgenA9.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Fixes: 13e6f2da ("clk: st: STiH407: Support for A9 MUX Clocks")
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>