rt2x00: rt2800lib: turn on tertiary PAs/LNAs for 3T/3R devices

The 3T/3R devices are using the tertiary PAs/LNAs
however those are never turned on. Fix the code to
turn on those on for such devices.

Also modify the code to use switch statements to
improve readability.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>