mlxsw: spectrum: Add FID get / set functions

As previously explained, not all vPorts will be assigned FIDs, so instead
of returning the FID index of a vPort, return a pointer to its FID
struct. This will allow us to know whether it's legal to access the
vPort's FID parameters such as index and device.

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>