mmc: atmel-mci: conform to DMA-API

Fixes the following:
- It is perfectly legal for the dma_map_sg() to return fewer
  entries than were passed in.
- Supply the returned numer of (possibly coalesced) entries to
  the device_pre_slave_sg() function.
- Use the proper original sg_len when unmapping the sglist
  in the error path.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>