net: dsa: b53: Rename num_arl_entries to num_arl_bins

The variable currently holds the number of ARL bins per ARL buckets,
which is different from the number of ARL entries which would be bins
times buckets. We will be adding a num_arl_buckets in a subsequent patch
so get variables straight now.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>