iwlwifi: fix rate-scaling algorithm for BT combo devices

iwlwifi tries to avoid using antenna B in BT combo devices when BT
is active. A bug in the rate-scaling algorithm was causing the combo
device to never attempt MIMO rates. Fix the algorithm to
opportunistically try MIMO rates when BT traffic is low.

Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>