macvlan: don't broadcast PAUSE frames to macvlan devices

PAUSE frames are only relevant for the real device, broadcasting them
to all macvlan devices can cause a significant load increase.

Reported-by: Ben Greear <greearb@candelatech.com>
Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>