blackfin: replace bfin_gen_rand_mac() with eth_random_addr()

bfin_gen_rand_mac() uses __DATE__ as the seed for random ethernet
address. This makes the build non-deterministic.

In the first place, it should not be implemented as a Bfin-specific
function. Use eth_random_addr() instead.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Sonic Zhang <sonic.zhang@analog.com>