net: systemport: Fix software statistics for SYSTEMPORT Lite

With SYSTEMPORT Lite we have holes in our statistics layout that make us
skip over the hardware MIB counters, bcm_sysport_get_stats() was not
taking that into account resulting in reporting 0 for all SW-maintained
statistics, fix this by skipping accordingly.

Fixes: 44a4524c ("net: systemport: Add support for SYSTEMPORT Lite")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>