mmc: mxs-mmc: Fix warning due to incorrect type

Fixes the following warning when building with W=1 option:

drivers/mmc/host/mxs-mmc.c: In function 'mxs_mmc_adtc':
drivers/mmc/host/mxs-mmc.c:401:2: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

The warning happens because 'i' is used in 'for_each_sg(sgl, sg, sg_len, i)' and should be made unsigned.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>