net: deprecate print_mac

We've had %pM for long enough now, time to deprecate
print_mac() and remove the __maybe_unused attribute
from DECLARE_MAC_BUF so that variables declared with
that can be found and removed. Otherwise people are
putting in new users of print_mac().

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>