mmc: mxs-mmc: disable regulator on error and in the remove function

The 'reg_vmmc' regulator is enabled in the probe. It is never disabled.
Neither in the error handling path of the probe nor in the remove
function.

Register a devm_action to disable it when needed.

Fixes: 4dc5a79f

 ("mmc: mxs-mmc: enable regulator for mmc slot")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/4aadb3c97835f7b80f00819c3d549e6130384e67.1634365151.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>