gianfar: Add device ref (dev) in gfar_private

gianfar: Add device ref (dev) in gfar_private

Use device pointer (dev) to simplify the code and to
avoid double indirections, especially on the hot path.

Basically, instead of accessing priv to get the ofdev
reference and then accessing the ofdev structure to
dereference the needed dev pointer, we will get the
dev pointer directly from priv.

The dev pointer is required on the hot path, see gfar_new_rxbdp
or gfar_clean_rx_ring (or xmit), and this patch makes
it available directly from priv's 1st cacheline.

This change is reflected at asm level too, taking (the hot)
gfar_new_rxbdp():
initial version -
    18c0:	7c 7e 1b 78 	mr      r30,r3

    18d0:	81 69 04 3c 	lwz     r11,1084(r9)

    18d8:	34 6b 00 10 	addic.  r3,r11,16
    18dc:	41 82 00 08 	beq-    18e4

patched version -
    18d0:	80 69 04 38 	lwz     r3,1080(r9)

    18d8:	2f 83 00 00 	cmpwi   cr7,r3,0
    18dc:	41 9e 00 08 	beq-    cr7,18e4

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>