tools/net/ynl: fix cli.py --subscribe feature

tools/net/ynl: fix cli.py --subscribe feature

Execution of command:
./tools/net/ynl/cli.py --spec Documentation/netlink/specs/dpll.yaml /
	--subscribe "monitor" --sleep 10
fails with:
  File "/repo/./tools/net/ynl/cli.py", line 109, in main
    ynl.check_ntf()
  File "/repo/tools/net/ynl/lib/ynl.py", line 924, in check_ntf
    op = self.rsp_by_value[nl_msg.cmd()]
KeyError: 19

Parsing Generic Netlink notification messages performs lookup for op in
the message. The message was not yet decoded, and is not yet considered
GenlMsg, thus msg.cmd() returns Generic Netlink family id (19) instead of
proper notification command id (i.e.: DPLL_CMD_PIN_CHANGE_NTF=13).

Allow the op to be obtained within NetlinkProtocol.decode(..) itself if the
op was not passed to the decode function, thus allow parsing of Generic
Netlink notifications without causing the failure.

Suggested-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/netdev/m2le0n5xpn.fsf@gmail.com/
Fixes: 0a966d606c68 ("tools/net/ynl: Fix extack decoding for directional ops")
Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20240904135034.316033-1-arkadiusz.kubalewski@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>