net: fec: cache statistics while device is down

Execution 'ethtool -S' on fec device that is down causes OOPS on Vybrid
board:

Unhandled fault: external abort on non-linefetch (0x1008) at 0xe0898200
pgd = ddecc000
[e0898200] *pgd=9e406811, *pte=400d1653, *ppte=400d1453
Internal error: : 1008 [#1] SMP ARM
...

Reason of OOPS is that fec_enet_get_ethtool_stats() accesses fec
registers while IPG clock is stopped by PM.

Fix that by caching statistics in fec_enet_private. Cache is initialized
at device probe time, and updated at statistics request time if device
is up, and also just before turning device off on down path.

Additional locking is not needed, since cached statistics is accessed
either before device is registered, or under rtnl_lock().

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>