Similar to the *_get_mac_binary function, also split the common parts
off mtd_get_mac_ascii into new get_mac_ascii function and introduce
mmc_get_mac_ascii which uses it.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 45c85c1827d45f8041b8f270d80bf6fff328069c)