iwlagn: fix needed chains calculation

Garen noticed that this was wrong. Fix
the calibration -- default to multiple
chains and fall back to single where
possible.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>