net: dsa: qca8k: introduce qca8k_bulk_read/write function

Introduce qca8k_bulk_read/write() function to use mgmt Ethernet way to
read/write packet in bulk. Make use of this new function in the fdb
function and while at it reduce the reg for fdb_read from 4 to 3 as the
max bit for the ARL(fdb) table is 83 bits.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>