iwlwifi: rename 6000 series Gen2 devices to Gen2a

Rename the current 6000 series Gen2 devices to Gen2a.
Rename the ucode name prefix to iwlwifi-6000g2a.
Also corrected the device IDs for Gen2a series devices.

Signed-off-by: Jay Sternberg <jay.e.sternberg@intel.com>
Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>