ath9k: fix PA predistortion thermal measurement handling

To be able to measure the thermal values correctly for PAPRD, we need
to send training frames before setting up the gain table for the measurement,
and then again afterwards for the actual training.

For further improvement, send training frames at MCS0 instead of 54 MBit/s
legacy. That way we can use the No-ACK flag for the transmission, which
speeds up PAPRD training in general, as the hardware won't have to
retransmit and wait for ACK timeout (was previously set to 4 * 6
transmission attempts).

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>