iwlwifi: read enhanced tx power info from EEPROM image

For 6000 series and up, additional enhanced regulatory tx power
limitation information is added to EEPROM image.

In order to setup the tx power limitation per channel correctly. Read
the enhanced tx power information from EEPROM image and update
accordingly.

The information is provided per SISO (a,b,c) chain based, it also has
information for both MIMO2 and MIMO3. For tx power regulatory
limitation, take the highest number from all the chains and update.
Also update tx_power_user_lmt to the highest power supported by any
channels and chains

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>