mlxsw: spectrum_router: Reuse work neighbor initialization in work scheduler

After the struct mlxsw_sp_netevent_work.n field initialization is moved
here, the body of code that handles NETEVENT_NEIGH_UPDATE is almost
identical to the one in the helper function. Therefore defer to the helper
instead of inlining the equivalent.

Note that previously, the code took and put a reference of the netdevice.
The new code defers to mlxsw_sp_dev_lower_is_port() to obviate the need for
taking the reference.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>