mlxsw: spectrum_router: Allow returning errors from mlxsw_sp_nexthop_group_refresh()

The function is responsible for allocating the adjacency entries used by
the nexthop group and populating them with the adjacency information
such as egress RIF and MAC address.

Allow the function to return an error when it encounters a problem and
have the relevant call sites check it.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>