sfc: Avoid NULL pointer dereference on systems without numa awareness

sfc: Avoid NULL pointer dereference on systems without numa awareness

On such systems cpumask_of_node() returns NULL, which bitmap
operations are not happy with.

Fixes: c265b569a45f ("sfc: default config to 1 channel/core in local NUMA node only")
Fixes: 09a99ab16c60 ("sfc: set affinity hints in local NUMA node only")
Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Reviewed-by: Íñigo Huguet <ihuguet@redhat.com>
Link: https://lore.kernel.org/r/164857006953.8140.3265568858101821256.stgit@palantir17.mph.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>