net: systemport: fix software maintained statistics

Commit 60b4ea17 ("net: systemport: log RX buffer allocation and RX/TX DMA
failures") added a few software maintained statistics using
BCM_SYSPORT_STAT_MIB_RX and BCM_SYSPORT_STAT_MIB_TX. These statistics are read
from the hardware MIB counters, such that bcm_sysport_update_mib_counters() was
trying to read from a non-existing MIB offset for these counters.

Fix this by introducing a special type: BCM_SYSPORT_STAT_SOFT, similar to
BCM_SYSPORT_STAT_NETDEV, such that bcm_sysport_get_ethtool_stats will read from
the software mib.

Fixes: 60b4ea17

 ("net: systemport: 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>