mlxsw: spectrum_router: Set FIB entry's type based on nexthop group

The previous patch associated a nexthop group with the FIB entry before
the entry's type is determined.

Make use of the nexthop group when determining the entry's type instead
of relying on helpers that assume that the nexthop info is not a nexthop
object (i.e., 'struct nexthop').

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