rt2x00: reduce tx power to nominal level on RT6352

Current implementation of RT6352 support provides too high tx power
at least on iPA/eLNA devices. Reduce amplification of variable gain
amplifier by 6dB to match board target power of 17dBm.
Transmited signal strength with this patch is similar to that of
stock firmware or pandorabox firmware. Throughput measured with iperf
improves. Device tested: Xiaomi Miwifi Mini.

Signed-off-by: Tomislav Požega <pozega.tomislav@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>