net: bcmgenet: cleanup for bcmgenet_xmit_frag()

Add frag_size = skb_frag_size(frag) and use it when needed.

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