bridge: Allow clearing of pvid and untagged bitmap

Currently, it is possible to modify the vlan filter
configuration to add pvid or untagged support.
For example:
  bridge vlan add vid 10 dev eth0
  bridge vlan add vid 10 dev eth0 untagged pvid

The second statement will modify vlan 10 to
include untagged and pvid configuration.
However, it is currently impossible to go backwards
  bridge vlan add vid 10 dev eth0 untagged pvid
  bridge vlan add vid 10 dev eth0

Here nothing happens.  This patch correct this so
that any modifiers not supplied are removed from
the configuration.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>