bridge: use __set_bit in __br_vlan_set_default_pvid

The same optimization as the one in commit cc0be1ad ("net:
bridge: Slightly optimize 'find_portno()'") is needed for the
'changed' bitmap in __br_vlan_set_default_pvid().

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/4e35f415226765e79c2a11d2c96fbf3061c486e2.1637782773.git.lucien.xin@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>