ASoC: ep93xx-i2s: use devm_* helpers to cleanup probe

Use the devm_* helpers to cleanup the probe routine. This also eliminates
having to carry the mem value in the private data for the remove.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Tested-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>