mac80211: fix RX chains configuration

If the driver doesn't support 40 MHz channels, then
mac80211 erroneously sets number of RX chains to one
although the number of chains is independent of the
support for 40 MHz channels.

Fix this by checking the 40 MHz support only for the
code that sets the 40 MHz channel not the complete
HT code block.

This also means the HT20 channel type will always be
set in the changed code block so there's no need to
set it in case we override the AP due to invalid IEs
in the probe response/beacon.

The indentation is a bit quirky, but I'm rewriting
this code for VHT support so this will change again
very soon.

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