iwlagn: fix iwlagn_check_needed_chains

This function was intended to calculate the
number of RX chains needed, but could only
work where the AP's streams were asymmetric,
i.e. 2 TX and 3 RX or similar. In the case
where IEEE80211_HT_MCS_TX_RX_DIFF was not
set, this function would calculate the wrong
information.

Additionally, mac80211 didn't pass through
the required values at all, so it couldn't
work anyway.

Rewrite the logic in this function and add
appropriate comments to make it readable.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>