gianfar: Fix typo bug introduced by move to udp_hdr()

In commit 4bedb452 both the udp and tcp
cases where changed to use udp_hdr() instead of leaving the tcp case
alone and fixing with tcp_hdr().

This ended up causing random behavior with TCP connections because
of looking for tcp_hdr()->check in the wrong place.
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

drivers/net/gianfar.c

@@ -944,7 +944,7 @@ static inline void gfar_tx_checksum(struct sk_buff *skb, struct txfcb *fcb)
 		flags |= TXFCB_UDP;
 		fcb->phcs = udp_hdr(skb)->check;
 	} else
-		fcb->phcs = udp_hdr(skb)->check;
+		fcb->phcs = tcp_hdr(skb)->check;
 
 	/* l3os is the distance between the start of the
 	 * frame (skb->data) and the start of the IP hdr.