tools: ynl-gen: fix parse multi-attr enum attribute

When attribute is enum type and marked as multi-attr, the netlink
respond is not parsed, fails with stack trace:
Traceback (most recent call last):
  File "/net-next/tools/net/ynl/./test.py", line 520, in <module>
    main()
  File "/net-next/tools/net/ynl/./test.py", line 488, in main
    dplls=dplls_get(282574471561216)
  File "/net-next/tools/net/ynl/./test.py", line 48, in dplls_get
    reply=act(args)
  File "/net-next/tools/net/ynl/./test.py", line 41, in act
    reply = ynl.dump(args.dump, attrs)
  File "/net-next/tools/net/ynl/lib/ynl.py", line 598, in dump
    return self._op(method, vals, dump=True)
  File "/net-next/tools/net/ynl/lib/ynl.py", line 584, in _op
    rsp_msg = self._decode(gm.raw_attrs, op.attr_set.name)
  File "/net-next/tools/net/ynl/lib/ynl.py", line 451, in _decode
    self._decode_enum(rsp, attr_spec)
  File "/net-next/tools/net/ynl/lib/ynl.py", line 408, in _decode_enum
    value = enum.entries_by_val[raw].name
TypeError: unhashable type: 'list'
error: 1

Redesign _decode_enum(..) to take a enum int value and translate
it to either a bitmask or enum name as expected.

Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20230725101642.267248-3-arkadiusz.kubalewski@intel.com
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>