wifi: iwlwifi: mvm: add EHT A-MPDU size exponent support

Add support for reading the EHT MAC capabilities A-MPDU
size exponent field, as indicated by the draft spec.

Also clarify the existing code a bit and add comments
so it's clearer to understand what's going on here.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230620125813.c5e00045d90f.I7520787fca8f8430a564adedf975d069ad8c5417@changeid


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