mmc: atmel-mci: fix a potential issue about pending PDC interrupts

mmc: atmel-mci: fix a potential issue about pending PDC interrupts

This patch fixes a potential issue about PDC interrupts. For example we
have a ENDRX pending interrupt and a RXBUFF pending interrupt. We have
received the RXBUFF interrupt but the transfer is not finished (so we
didn't have time to give a new buffer to the PDC controller). Then we
will compute ENDRX interrupt and we will give a new buffer to the PDC
controller, just after we will compute the RXBUFF interrupt and give
one or two new buffers to the PDC controller but we are not sure that
the first buffer given has been filled. So in this situation we may
have "lost" one sg buffer. It's the same for transmission.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>