net: bgmac: Pad packets to a minimum size

In preparation for enabling Broadcom tags with b53, pad packets to a
minimum size of 64 bytes (sans FCS) in order for the Broadcom switch to
accept ingressing frames. Without this, we would typically be able to
DHCP, but not resolve with ARP because packets are too small and get
rejected by the switch.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>