iwlwifi: Distinguish power amplifier for 6000 series

For 6x00 2x2 NIC, two types of Power Amplifier are available.
In order for uCode to apply correct tx power,
driver needs to program the CSR_GP_DRIVER_REG register and
let uCode know the type of PA.
If driver do not program CSR_GP_DRIVER_REG register (default to 0),
then it is uCode's decision for tx power

2x2 Hybrid card: use both internal and external PA
2x2 IPA(Internal Power Amplifier) card: internal PA only

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>