iwlwifi: set AMPDU status variables correctly

The TX status code is currently abusing the ampdu_ack_map field (a bitmap) to
count the number of successfully received frames.  The comments in mac80211.h
show there are actually three different, relevant variables, of which we are
currently using two, both incorrectly. Fix this by making

- ampdu_ack_len -> the number of ACKed frames (i.e. successes)
- ampdu_ack_map -> the bitmap
- ampdu_len -> the total number of frames sent (i.e., attempts)

to match the header file (and verified with ath9k's usage) and updating Intel's
RS code to match.

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>