iwlwifi: fix max_ht_ampdu_exponent for older devices

The commit below didn't update the max_ht_ampdu_exponent
for the devices listed in iwl-[1-6]000.c which, in result,
became 0 instead of 8K. This reduced the size of the Rx
AMPDU from 64K to 8K which had an impact in the Rx
throughput. One user reported that because of this, his
downstream throughput droppped by a half.

CC: <stable@vger.kernel.org> [3.19]
Fixes: c064ddf3

 ("iwlwifi: change max HT and VHT A-MPDU exponent")
Reported-and-tested-by: Valentin Manea <linux-wireless@mrs.ro>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>