net: dsa: b53: Rework ARL bin logic

[ Upstream commit 6344dbde ]

When asking the ARL to read a MAC address, we will get a number of bins
returned in a single read. Out of those bins, there can essentially be 3
states:

- all bins are full, we have no space left, and we can either replace an
  existing address or return that full condition

- the MAC address was found, then we need to return its bin index and
  modify that one, and only that one

- the MAC address was not found and we have a least one bin free, we use
  that bin index location then

The code would unfortunately fail on all counts.

Fixes: 1da6df85

 ("net: dsa: b53: Implement ARL add/del/dump operations")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>