mlxsw: spectrum: Use join / leave functions for vFID operations

When a vPort is created or when it joins a bridge we always do the same
set of operations:

1) Create the vFID, if not already created
2) Setup flooding for the vFID
3) Map the {Port, VID} to the vFID

When a vPort is destroyed or when it leaves a bridge the reverse is
performed.

Encapsulate the above in join / leave functions and simplify the code.
FIDs and rFIDs will use a similar set of functions.
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>