Revert "imx_common: Return MMCSD_MODE_FS in spl_boot_mode() also for EXTFS"

Revert "imx_common: Return MMCSD_MODE_FS in spl_boot_mode() also for EXTFS"

Commit c1ebf54868359005 ("imx_common: Return MMCSD_MODE_FS in spl_boot_mode()
also for EXTFS") causes SPL breakage on wandboard:

ERROR: v7_dcache_inval_range - start address is not aligned - 0x1820006c
ERROR: v7_dcache_inval_range - stop address is not aligned - 0x1820086c
ERROR: v7_dcache_inval_range - start address is not aligned - 0x1820006c
ERROR: v7_dcache_inval_range - stop address is not aligned - 0x1820086c
** First descriptor is NOT a primary desc on 0:1 **
spl: no partition table found
SPL: failed to boot from all boot devices
### ERROR ### Please RESET the board ###

This error is seen when SPL and u-boot.img are stored in the raw SD card
partition.

This reverts commit c1ebf54868359005c32944c1473668d5fcaca158.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>