net: dsa: sja1105: delete vlan delta save/restore logic

With the best_effort_vlan_filtering mode now gone, the driver does not
have 3 operating modes anymore (VLAN-unaware, VLAN-aware and best effort),
but only 2.

The idea is that we will gain support for network stack I/O through a
VLAN-aware bridge, using the data plane offload framework (imprecise RX,
imprecise TX). So the VLAN-aware use case will be more functional.

But standalone ports that are part of the same switch when some other
ports are under a VLAN-aware bridge should work too. Termination on
those should work through the tag_8021q RX VLAN and TX VLAN.

This was not possible using the old logic, because:
- in VLAN-unaware mode, only the tag_8021q VLANs were committed to hw
- in VLAN-aware mode, only the bridge VLANs were committed to hw
- in best-effort VLAN mode, both the tag_8021q and bridge VLANs were
  committed to hw

The strategy for the new VLAN-aware mode is to allow the bridge and the
tag_8021q VLANs to coexist in the VLAN table at the same time.

[ yes, we need to make sure that the bridge cannot install a tag_8021q
  VLAN, but ]

This means that the save/restore logic introduced by commit ec5ae610


("net: dsa: sja1105: save/restore VLANs using a delta commit method")
does not serve a purpose any longer. We can delete it and restore the
old code that simply adds a VLAN to the VLAN table and calls it a day.

Note that we keep the sja1105_commit_pvid() function from those days,
but adapt it slightly. Ports that are under a VLAN-aware bridge use the
bridge's pvid, ports that are standalone or under a VLAN-unaware bridge
use the tag_8021q pvid, for local termination or VLAN-unaware forwarding.

Now, when the vlan_filtering property is toggled for the bridge, the
pvid of the ports beneath it is the only thing that's changing, we no
longer delete some VLANs and restore others.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>