Implement ReadRSSI for Linux

[denysvitali]

Currently, it is not possible to get the RSSI value of a connected device because this feature isn't implemented.

ble/linux/gatt/client.go

Lines 275 to 281 in 731710e

	// ReadRSSI retrieves the current RSSI value of remote peripheral. [Vol 2, Part E, 7.5.4]
	func (p *Client) ReadRSSI() int {
	p.Lock()
	defer p.Unlock()
	// TODO:
	return 0
	}