iwlwifi: clarify and clean up chain settings

iwlwifi: clarify and clean up chain settings

The chain settings we currently use in iwlwifi are
rather confusing -- and we also go by the wrong
settings entirely under certain circumstances. To
clean it up, create a new variable in the current
HT config -- single_chain_sufficient -- that tells
us whether we need more than one chain. Calculate
that based on the AP and operating mode (no IBSS
HT implemented -- so no need for multiple chains,
for station mode we use the AP's capabilities).

Additionally, since APs always send disabled SM PS
mode, keeping track of their sm_ps mode isn't very
useful -- doubly not so for our _own_ RX config
since that should depend on our, not the AP's, SM
PS mode.

Finally, document that our configuration of the
number of RX chains used is currently wrong when
in powersave (by adding a comment).

All together this removes the two remaining items
in struct iwl_ht_config that were done wrong there.

For the future, the number of RX chains and some
SM PS handshaking needs to be added to mac80211,
which then needs to tell us, and the new variable
current_ht_config.single_chain_sufficient should
also be calculated by mac80211.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>