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 9f242738 ("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 4dc89133 ("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 f8ff080d ("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...