NET: pch, fix use after free

Stanse found that pch_gbe_xmit_frame uses skb after it is freed. Fix
that.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Masayuki Ohtake <masa-korg@dsn.okisemi.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>