rt2x00: rt2800lib: support for for RT3352 with external PA

This is needed for WiFi to work e.g. on DIR-615 rev.H1 which got
external RF power amplifiers connected to the WiSoC.

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