otRadioFrame::::::mTxPower field

The transmit power in dBm. If the platform layer does not provide `OT_RADIO_CAPS_TRANSMIT_FRAME_POWER` capability, it can ignore this value. If the value is OT_RADIO_POWER_INVALID, then the platform should ignore this value and transmit the frame with its default transmit power. Otherwise, the platform should transmit this frame with the maximum power no larger than minimal of the following values: 1. mTxPower, 2. The power limit set by otPlatRadioSetChannelTargetPower(), 3. The power limit set by otPlatRadioSetChannelMaxTransmitPower(), 4. The power limit set by otPlatRadioSetRegion().