iwlagn: rename enhanced txpower fields

Some fields we didn't previously use from the
enhanced TX power structure will be needed in
the next patch, so rename them to their correct
names to be able to use them and change code
reading them accordingly.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>