tools: ynl: fully inherit attrs in subsets

To avoid having to repeat the entire definition of an attribute
(including the value) use the Attr object from the original set.
In fact this is already the documented expectation.

Fixes: be5bea1c ("net: add basic C code generators for Netlink")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>