board: mediatek: fix mmc_get_boot_dev() for platforms without external SD

On the UniElec U7623 board there is no external SD slot and the preloader
doesn't fill in the magic field at 0x81dffff0 to indicate that it was
booted from eMMC.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
1 file changed