ofp-util: Fix OF1.4+ version of ofputil_decode_set_async_config().

ofp-util: Fix OF1.4+ version of ofputil_decode_set_async_config().

The OF1.0 through OF1.3 "set async config" set the whole configuration,
OF1.4+ only update parts of it piecemeal, but the decoding function always
set the whole configuration.  This commit fixes the problem by changing the
interface to require the caller to provide an initial state.  (It would
be possible to simply make it mutate existing state in-place, but that
interface seems a little more error-prone.)

Found by inspection.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>