mlxsw: spectrum_router: Avoid missing error code warning

Explicitly set the error code to zero before the goto statement to avoid
the following smatch warning:

drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:3598 mlxsw_sp_nexthop_group_refresh() warn: missing error code 'err'

The warning is a false positive, but the change both suppresses the
warning and makes it clear to future readers that this is not an error
path.

The original report and discussion can be found here [1].

[1] https://lore.kernel.org/lkml/202105141823.Td2h3Mbi-lkp@intel.com/

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>