imx_common: Return MMCSD_MODE_FS in spl_boot_mode() also for EXTFS

imx_common: Return MMCSD_MODE_FS in spl_boot_mode() also for EXTFS

spl_boot_mode() returned MMCSD_MODE_RAW on MMC if CONFIG_SPL_EXT_SUPPORT
was configured. EXTFS is the default filesystem selected in imx6_spl.h
and the function should return MMCSD_MODE_FS instead.

Fix this and return MMCSD_MODE_FS instead in such cases.

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
CC: Stefano Babic <sbabic@denx.de>
CC: Tim Harvey <tharvey@gateworks.com>
CC: Fabio Estevam <Fabio.Estevam@freescale.com>