mmc: atmel-mci: correct sg buffer size evaluation

Assuming that a sg buffer size is a page size is false so use sg_dma_len.
A 4096 bytes can be required with two 2048-bytes sg buffer.

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>