mlxsw: spectrum_switchdev: Don't batch learning operations

We no longer batch VLAN operations, so there's no need to set the
learning state for a range of VLANs.

Use a common function to set the learning state for a Port-VLAN, thereby
making the code saner more receptive for upcoming changes.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>