BQR: Correct the type of tx_power_level to signed int.

The parameter description of the TX_Power_Level:
* Current transmit power level for the specified Connection_Handle. This value shall be the same as the controller's responding to the HCI_Read_Transmit_Power_Level HCI command.
* Size: 1 octet (signed integer)
* Units: dBm

Bug: 156127397
Tag: #feature
Test: manual
Change-Id: Ife67ebd83669af80e6c91c04c893f6f624894eea