clk_prepare_enable() may fail, so let's check its return value and propagate it
in the case of error.

While at it, fix the order of clk_disable_unprepare calls: clk_ipg should be
disabled first, followed by clk_per.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>