mmc: jz4740: Fix handling of read errors.

For no reason, the code handling write errors was implemented while
the code handling read errors was missing.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Chris Ball <cjb@laptop.org>