iwlwifi: pcie: adjust to Bz completion descriptor

The Bz devices got a new completion descriptor again since
we only ever really used 4 out of 32 bytes anyway. Adjust
the code to deal with that. Note that the intention was to
reduce the size, but the hardware was implemented wrongly.

While at it, do some cleanups and remove the union to simplify
the code, clean up iwl_pcie_free_bd_size() to no longer need
an argument and add iwl_pcie_used_bd_size() with the logic to
selct completion descriptor size.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20220204122220.bef461a04110.I90c8885550fa54eb0aaa4363d322f50e301175a6@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>