mx6qsabresd: Fix booting the kernel from SDHC3

Since commit de7d02ae (mx6qsabresd: add usdhc2 and usdhc4 support)
SDHC3 device node is no longer 0, which breaks loading a uImage from SDHC3.

Fix it by adapting the default environment to use CONFIG_SYS_MMC_ENV_DEV as the
correct mmc node for loading the kernel from.

While at it, go back to using SDHC3 as the default mmc, since we have Yocto
images that generate an SD card containing U-boot,kernel and rootfs, so it is
more convenient to keep using SDHC3 as it was originally.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>