mlxsw: spectrum: Don't assume upper device's type

When an upper device is configured on top of a vPort we make sure it's a
bridge master during PRECHANGEUPPER and fail otherwise. Therefore, when
CHANGEUPPER is later received we don't bother checking the upper's type.

Make the code more extendable in preparation for VRF uppers, by checking
the upper's type.

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>