Using a temp. buffer for a disk sector on the stack, seems not work. Doing so
lets the system run crazy (the stack seems to be destroyd). Don't know the
correct stack handling on ARM, but (IMHO) I also can exclude any writing across
the buffer boundaries.

Using a temp. buffer via malloc() runs also on ARM.

Note: This patch was required to add MCI card support, which can be handled
like a disk drive.

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>