base-files: use get_mac_binary() in mtd_get_mac_binary_ubi()

base-files: use get_mac_binary() in mtd_get_mac_binary_ubi()

The actual retrieval of the MAC address in mtd_get_mac_binary_ubi()
is the same as in get_mac_binary(). Thus, use the latter function
in the former to reduce duplicate code.

This will also allow to benefit from the enhanced path check there
and bring mtd_get_mac_binary_ubi() more in line with the similar
mtd_get_mac_binary().

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>