atmel-mci: Don't overwrite error bits when NOTBUSY is set

After a data error, we wait for the NOTBUSY bit to be set so that we can
be sure the data transfer is completely finished. However, when NOTBUSY
is set, the interrupt handler copies the contents of SR into
data_status, overwriting any error bits we may have detected earlier.

To avoid this, initialize data_status to 0 before starting a request, and
don't overwrite it unless it still contains 0.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>