iwlagn: fix warning in testmode attribute table

Fix the compile warning cause by [IWL_TM_ATTR_MAX - 1]

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