net: dsa: sja1105: Receive and decode meta frames

This adds support in the tagger for understanding the source port and
switch id of meta frames.  Their timestamp is also extracted but not
used yet - this needs to be done in a state machine that modifies the
previously received timestampable frame - will be added in a follow-up
patch.

Also take the opportunity to:
- Remove a comment in sja1105_filter made obsolete by e8d67fa5
  ("net: dsa: sja1105: Don't store frame type in skb->cb")
- Reorder the checks in sja1105_filter to optimize for the most likely
  scenario first: regular traffic.

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