rt2x00: reverse external PA capability flag logic

Consequently refer to external PA instead of inverting the logic and
use an internal PA capability flag which is a bit confusing.
Currently this is used for Rt3352 only, but MT7620A also allows for an
external PA which will be supported by a follow up patch.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>