ARM: imx: refactor mxc_iomux_mode()

Refactor mxc_iomux_mode():
 - since it always returns 0 make it to return void
 - remove unnecessary ret variable
 - declare variables according to the kernel coding style

Signed-off-by: Dmitry Voytik <voytikd@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>