net: bcmgenet: correct MIB access of UniMAC RUNT counters

The gap between the Tx status counters and the Rx RUNT counters is now
being added to allow correct reporting of the registers.

Fixes: 1c1008c7 ("net: bcmgenet: add main driver file")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>