iwlwifi: "is_fat" bit in rate scale match RXON flag

This patch change the "is_fat" checking in rate scale to use
iwl_is_fat_tx_allowed() to match the sta and RX_ON command setting.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Tested-by: Conrad Kostecki <ConiKost@gmx.de>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>