net: bgmac: Remove short packet padding for DSA

DSA now correctly pads short packets within net/dsa/tag_brcm.c such that
this it is no longer necessary to do this within bgmac.

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