[ETHER]: Bring back MAC_FMT

The print_mac function is not very suitable for debugging printks
in performance critical paths since without ifdefs it will always
get called. MAC_FMT can be used with pr_debug without any overhead
when debugging is disabled.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>