gianfar: Fix and cleanup Rx FCB indication

gianfar: Fix and cleanup Rx FCB indication

This fixes a less obvious error on one hand, and prevents futher
similar errors by disambiguating and optimizing RxFCB indication,
on the other hand.

The error consists in NETIF_F_HW_VLAN_TX flag being used as an
indication of Rx FCB insertion. This happened as soon gfar_uses_fcb(),
which despite its name indicates Rx FCB insertion, started
incorporating is_vlan_on().
is_vlan_on(), on the other hand, is also a misleading construct because
we need to differentiate b/w hw VLAN extraction/VLEX (marked by VLAN_RX
flag) and hw VLAN insertion/VLINS (VLAN_TX flag), which are different
mechanisms using different types of FCBs.

The hw spec for the RxFCB feature is as follows:
In the case of RxBD rings, FCBs (Frame Control Block) are inserted by
the eTSEC whenever RCTRL[PRSDEP] is set to a non-zero value. Only one
FCB is inserted per frame (in the buffer pointed to by the RxBD with
bit F set). TOE acceleration for receive is enabled for all rx frames
in this case.

This patch introduces priv->uses_rxfcb field to quickly signal RxFCB
insertion in accordance with the specification above.

The dependency on FSL_GIANFAR_DEV_HAS_TIMER was also eliminated as
another source of confusion. The actual dependency is to priv->hwts_rx_en.
Upon changing priv->hwts_rx_en via IOCTL, the gfar device is being
restarted and on init_mac() the priv->hwts_rx_en flag determines RxFCB
insertion, and rctrl is programmed accordingly. The patch takes care
of this case too.

Though maybe not as self documenting as the inlining version uses_fcb(),
priv->uses_rxfcb has the main purpose to quickly signal, on the hot path,
that the incoming frame has a *Rx* FCB block inserted which needs to be
pulled out before passing the skb to the stack. This is a performance
critical operation, it needs to happen fast, that's why uses_rxfcb is
placed in the first cacheline of gfar_private.
This is also why a cached rctrl cannot be used instead: 1) because
we don't have 32 bits available in the first cacheline of gfar_priv
(but only 16); 2) bit operations are expensive on the hot path.

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