iwlwifi: make sure reduced tx power bit is valid

Once driver decide to change reduced tx power behavior,
make sure the reduce tx power valid bit is set

Change-Id: I3afae96319292d8cb347a812a948085c5db7ad91
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Reviewed-on: http://git-mwg.jer.intel.com/gerrit/1948


Tested-by: Jenkins
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>