net: Remove usage of net_device last_rx member

The network stack no longer uses the last_rx member of struct net_device
since the bonding driver switched to use its own private last_rx in
commit 9f24273837 ("bonding: use last_arp_rx in slave_last_rx()").

However, some drivers still (ab)use the field for their own purposes and
some driver just update it without actually using it.

Previously, there was an accompanying comment for the last_rx member
added in commit 4dc89133f4 ("net: add a comment on netdev->last_rx")
which asked drivers not to update is, unless really needed. However,
this commend was removed in commit f8ff080dac ("bonding: remove
useless updating of slave->dev->last_rx"), so some drivers added later
on still did update last_rx.

Remove all usage of last_rx and switch three drivers (sky2, atp and
smc91c92_cs) which actually read and write it to use their own private
copy in netdev_priv.

Compile-tested with allyesconfig and allmodconfig on x86 and arm.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Jay Vosburgh <j.vosburgh@gmail.com>
Cc: Veaceslav Falico <vfalico@gmail.com>
Cc: Andy Gospodarek <andy@greyhouse.net>
Cc: Mirko Lindner <mlindner@marvell.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/m68k/emu/nfeth.c

@@ -124,7 +124,6 @@ static inline void recv_packet(struct net_device *dev)
 
 	skb->protocol = eth_type_trans(skb, dev);
 	netif_rx(skb);
-	dev->last_rx = jiffies;
 	dev->stats.rx_packets++;
 	dev->stats.rx_bytes += pktlen;