mlxsw: spectrum_router: Fix error handling issue

Return error to the caller instead of suppressing it.

Fixes: e3ddfb45

 ("mlxsw: spectrum_router: Allow returning errors from mlxsw_sp_nexthop_group_refresh()")
Addresses-Coverity: ("Error handling issues  (CHECKED_RETURN)")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>