mt76: fix tx power issues

- tx power is stored in the channels after ieee80211_register_hw, so
  chan->orig_mpwr needs to be updated as well
- for non-TSSI devices, mt76x2e needs to use a different target power value
  from the EEPROM
- fix a rounding error in a few places (need to round up, not down)

Signed-off-by: Felix Fietkau <nbd@nbd.name>