net: dsa: tag_sja1105: fix control packets on SJA1110 being received on an imprecise port

On RX, a control packet with SJA1110 will have:
- an in-band control extension (DSA tag) composed of a header and an
  optional trailer (if it is a timestamp frame). We can (and do) deduce
  the source port and switch id from this.
- a VLAN header, which can either be the tag_8021q RX VLAN (pvid) or the
  bridge VLAN. The sja1105_vlan_rcv() function attempts to deduce the
  source port and switch id a second time from this.

The basic idea is that even though we don't need the source port
information from the tag_8021q header if it's a control packet, we do
need to strip that header before we pass it on to the network stack.

The problem is that we call sja1105_vlan_rcv for ports under VLAN-aware
bridges, and that function tells us it couldn't identify a tag_8021q
header, so we need to perform imprecise RX by VID. Well, we don't,
because we already know the source port and switch ID.

This patch drops the return value from sja1105_vlan_rcv and we just look
at the source_port and switch_id values from sja1105_rcv and sja1110_rcv
which were initialized to -1. If they are still -1 it means we need to
perform imprecise RX.

Fixes: 884be12f ("net: dsa: sja1105: add support for imprecise RX")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>