tools: ynl: default to treating enums as flags for mask generation

I was a bit too optimistic in commit bf51d277

 ("tools: ynl: fix
get_mask utility routine"), not every mask we use is necessarily
coming from an enum of type "flags". We also allow flipping an
enum into flags on per-attribute basis. That's done by
the 'enum-as-flags' property of an attribute.

Restore this functionality, it's not currently used by any in-tree
family.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>