s390/qeth: gather more detailed RX dropped/error statistics

Where available, use the fine-grained counters in rtnl_link_stats64 to
indicate different RX error causes. For drop reasons, use driver-private
ethtool counters.

In particular this patch allows us to keep track of driver-side drops due
to unknown/unsupported HW descriptor format.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>