This patch brings no functional changes.
It moves out the VLAN code iterating on a list of VLAN members into new
dsa_switch_vlan_{prepare,add}_bitmap() functions.

This gives us a better isolation of the two switchdev phases.
Signed-off-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>