mlxsw: spectrum_nve: Make tunnel initialization symmetric

The device supports a single VTEP whose configuration is shared between
all VXLAN tunnels.

While the shared configuration is cleared upon the destruction of the
last tunnel - in mlxsw_sp_nve_tunnel_fini() - it is set in
mlxsw_sp_nve_fid_enable(), after calling mlxsw_sp_nve_tunnel_init().

Make tunnel initialization and destruction symmetric and set the
configuration in mlxsw_sp_nve_tunnel_init().

This will later allow us to protect the shared configuration with a
lock.

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