mlxsw: spectrum_router: Set FIB entry's type after creating nexthop group

Each FIB entry has a type (e.g., remote, local) that determines how the
entry is programmed to the device. In order to determine if the entry is
local (directly connected) or remote (has a gateway) the relevant FIB
info structures (e.g., 'struct fib_info') are checked.

When entries that use nexthop objects are supported, these checks will
need to be changed to take into account 'struct nexthop'.

Instead, first associate the entry with a nexthop group so that the next
patch could determine the entry's type based on the associated nexthop
group's type.

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