net: bcmgenet: correct the RBUF_OVFL_CNT and RBUF_ERR_CNT MIB values

The location of the RBUF overflow and error counters has moved between
different version of the GENET MAC.  This commit corrects the driver to
read from the correct locations depending on the version of the GENET
MAC.

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>