Current ksz8_fdb_dump() is able to extract only max 249 entries on
the ksz8863/ksz8873 series of switches. This happened due to wrong
bit mask and offset calculation.

This commit corrects the issue and allows for the complete extraction of
all 1024 entries.

Fixes: 4b20a07e ("net: dsa: microchip: ksz8795: add support for ksz88xx chips")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>