net: dsa: sja1105: Implement is_static for FDB entries on E/T

The first generation switches don't tell us through the dynamic config
interface whether the dumped FDB entries are static or not (the LOCKEDS
bit from P/Q/R/S).

However, now that we're keeping a mirror of all 'bridge fdb' commands in
the static config, this is an opportunity to compare a dumped FDB entry
to the driver's private database.  After all, what makes an entry static
is that *we* added it.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>