net: fec: Factorize the .xmit transmit function

Make the code more readable and easy to support other features like
SG, TSO, moving the common transmit function to one api.

And the patch also factorize the getting BD index to it own function.

CC: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec.h

@@ -296,6 +296,7 @@ struct fec_enet_private {
 	/* The ring entries to be free()ed */
 	struct bufdesc	*dirty_tx;
 
+	unsigned short bufdesc_size;
 	unsigned short tx_ring_size;
 	unsigned short rx_ring_size;