ice: make use of DEFINE_FLEX() for struct ice_aqc_dis_txq_item

Use DEFINE_FLEX() macro for 1-elem flex array use case
of struct ice_aqc_dis_txq_item.

Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://lore.kernel.org/r/20230912115937.1645707-7-przemyslaw.kitszel@intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>