net: bcmgenet: fix software maintained statistics

Commit 44c8bc3c ("net: bcmgenet: log RX buffer allocation and RX/TX dma
failures") added a few software maintained statistics using
BCMGENET_STAT_MIB_RX and BCMGENET_STAT_MIB_TX. These statistics are read from
the hardware MIB counters, such that bcmgenet_update_mib_counters() was trying
to read from a non-existing MIB offset for these counters.

Fix this by introducing a special type: BCMGENET_STAT_SOFT, similar to
BCMGENET_STAT_NETDEV, such that bcmgenet_get_ethtool_stats will read from the
software mib.

Fixes: 44c8bc3c ("net: bcmgenet: log RX buffer allocation and RX/TX dma failures")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>