imx: spl: return boot mode for asked MMC device in spl_mmc_boot_mode()

imx: spl: return boot mode for asked MMC device in spl_mmc_boot_mode()

Boards may extend or re-define the boot list in their board_boot_order()
function by modifying spl_boot_list. E.g. a board might boot SPL from a
slow SPI NOR flash and then load the U-Boot from an eMMC or SD-card.
Or it might use additional MMC boot device in spl_boot_list for cases
when the image in SPI NOR flash is not found, so it could fall back to
eMMC, SD-card or another boot device.

Getting the MMC boot mode in spl_mmc will fail when we are trying to
boot from an MMC device in the spl_boot_list and the original board
boot mode (as returned by spl_boot_device()) is not an MMC boot mode.
Fix it by checking the asked MMC boot device from the spl_mmc_boot_mode()
argument.

Signed-off-by: Anatolij Gustschin <agust@denx.de>