mmc: at91_mci: fix hanging and rework to match flowcharts

Fixes hanging using multi block operations (seen during CMD25).
Follows closely the datasheet flowcharts.

This piece of code handles better big file writing. I had to take care
of the notbusy signal during write (at91_mci_handle_cmdrdy function) and
to rearrange the AT91_MCI_ENDRX and AT91_MCI_RXBUFF flag usage.

Signed-off-by: Nicolas Ferre <nicolas.ferre@rfo.atmel.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>