mmc: mxs-mmc: Use devm_clk_get()

Using devm_clk_get() allows us to remove the clk_put() calls, so
let's use it to simplify the code.

Rename the 'out_clk_put' label to 'out_clk_disable' now that clk_put
is removed.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>