mfd: twl6040: Check clk_prepare_enable() return value in twl6040_power()

The clk_prepare_enable() function can fail so check the return
value and propagate the error in case of a failure.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>