mlxsw: spectrum: Move mlxsw_sp_span_gretap4_route()

Move the function next to the rest of gretap4 functions. Thus the
generic functions shared between gretap4 and gretap6 are in one block at
the beginning, followed by a gretap4 block, followed by a gretap6 block.

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