atmel_spi: fix warning In function 'atmel_spi_dma_map_xfer'

passing argument 2 of 'dma_map_single' discards qualifiers from pointer target type

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>