mlxsw: spectrum: Don't count on FID being present

Not all vPorts will have FIDs assigned to them, so make sure functions
first test for FID presence.

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>