mlxsw: spectrum_router: Fix wrong kfree() in error path

mlxsw: spectrum_router: Fix wrong kfree() in error path

The function allocates 'nhgi', not 'nh_grp', so it needs to free the
former in its error path.

Fixes: 7f7a417e6a11 ("mlxsw: spectrum_router: Split nexthop group configuration to a different struct")
Addresses-Coverity: ("Memory - corruptions  (USE_AFTER_FREE)")
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>