net: dsa: sja1105: Populate is_static for FDB entries on P/Q/R/S

The reason why this wasn't tackled earlier is that I had hoped I
understood the user manual wrong.  But unfortunately hacks are required
in order to retrieve the static/dynamic nature of FDB entries on SJA1105
P/Q/R/S, since this info is stored in the writeback buffer of the
dynamic config command.

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