mlxsw: spectrum: Create a function to map vPort's FID

A FID used by a vPort (vFID, but also rFID later in the series) is
always mapped using {Port, VID} and not only VID as with the 4K FIDs of
the VLAN-aware bridge.

Instead of specifying all the arguments each time, just wrap this
operation using a dedicated function and simplify the code.

As before, the function takes FID as its argument in preparation for a
generic FID struct.

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>