iwlwifi: validate enhanced tx power entry

Validate enhanced tx power entry read from EEPROM before applying the
tx power value. Different versions of EEPROM might contain different size
of table; always a good idea to make sure the entry is valid before
applying to the targeted channel.

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