net: bcmgenet: correctly pad short packets

Packets shorter than ETH_ZLEN were not padded with zeroes, hence leaking
potentially sensitive information. This bug has been present since the
driver got accepted in commit 1c1008c7


("net: bcmgenet: add main driver file").

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>