mlxsw: spectrum: Remove reference count from VLAN entries

Commit b3529af6 ("spectrum: Reference count VLAN entries") started
reference counting port-VLAN entries in a similar fashion to the 8021q
driver.

However, this is not actually needed and only complicates things.
Instead, the driver should forbid the creation of a VLAN on a port if
this VLAN already exists. This would also solve the issue fixed by the
mentioned commit.

Therefore, remove the get()/put() API and use create()/destroy()
instead.

One place that needs special attention is VLAN addition in a VLAN-aware
bridge via switchdev operations. In case the VLAN flags (e.g., 'pvid')
are toggled, then the VLAN entry already exists. To prevent the driver
from wrongly returning EEXIST, the driver is changed to check in the
prepare phase whether the entry already exists and only returns an error
in case it is not associated with the correct bridge port.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>