mlxsw: spectrum_span: Extract mlxsw_sp_span_entry_{de, }configure()

Configuring the hardware for encapsulated SPAN involves more code than
the simple mirroring case. Extract the related code to a separate
function to separate it from the rest of SPAN entry creation. Extract
deconfigure as well for symmetry, even though disablement is the same
regardless of SPAN type.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>