mmc: pxamci: fix the device-tree probe deferral path

When the gpio driver is probed after the mmc one, the read/write gpio
and card detection one return -EPROBE_DEFER. Unfortunately, the memory
region remains requested, and upon the next probe, the probe will fail
anyway with -EBUSY.

Fix this by releasing the memory resource upon probe failure.

More broadly, this patch uses devm_*() primitives whenever possible in
the probe function.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>