iwlwifi: fix Bz NMI behaviour

Contrary to what was stated before, the hardware hasn't changed
the bits here yet. In any case, the new CSR is also directly
(lower 16 bits) connected to UREG_DOORBELL_TO_ISR6, so if it
still changes the changes would be there. Adjust the code and
comments accordingly.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Fixes: 6c0795f1

 ("iwlwifi: implement Bz NMI behaviour")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20211210090244.75b6207536e3.I7d170a48a9096e6b7269c3a9f447c326f929b171@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>