uboot-imx

UBI: Fix build problem noticed on Apollon (arm/testing repo)

This patch fixes a build problem noticed on Apollon by using
mtd_dev_by_eb() instead of "/" as done in the Linux UBI version.
So this brings the U-Boot UBI version more in sync with the Linux
version again.

Signed-off-by: Stefan Roese <sr@denx.de>