ice: make use of DEFINE_FLEX() for struct ice_aqc_add_tx_qgrp

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

Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://lore.kernel.org/r/20230912115937.1645707-6-przemyslaw.kitszel@intel.com


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