ath10k: fix max antenna gain unit

Most of the txpower for the ath10k firmware is stored as twicepower (0.5 dB
steps). This isn't the case for max_antenna_gain - which is still expected
by the firmware as dB.

The firmware is converting it from dB to the internal (twicepower)
representation when it calculates the limits of a channel. This can be seen
in tpc_stats when configuring "12" as max_antenna_gain. Instead of the
expected 12 (6 dB), the tpc_stats shows 24 (12 dB).

Tested on QCA9888 and IPQ4019 with firmware 10.4-3.5.3-00057.

Fixes: 02256930 ("ath10k: use proper tx power unit")
Signed-off-by: Sven Eckelmann <seckelmann@datto.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20190611172131.6064-1-sven@narfation.org