mac80211: rt2x00: add incomplete support for external PA on MT7620

Do as the vendor driver does -- however, some devices apparently
patched support for external PA into the driver similar to how it was
done on Rt3352, using EEPROM_NIC_CONF1 rather than EEPROM_NIC_CONF2,
hence we check for both fields. Somehow the vendor driver also no
longer offers the option of only one of the TX paths having an external
PA (which was probably to weird to ever be implemented in practise,
though it doesn't seem like a particularly bad idea to me). Do the same
in rt2x00 and enable support for external PA on both TX paths whenever
it is set for TX0 in EEPROM.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>