ReadLongCharacteristic() uses RxMtu()

[sschulz-t]

I think ReadLongCharacteristic() should use RxMTU() to decide if the read length hints to a follow up transfer:

ble/linux/gatt/client.go

Lines 234 to 239 in 8c5522f

	for len(read) >= p.conn.TxMTU()-1 {
	if read, err = p.ac.ReadBlob(c.ValueHandle, uint16(len(buffer))); err != nil {
	return nil, err
	}
	buffer = append(buffer, read...)
	}

Testcase:
Call client.ExchangeMTU(200) to limit the MTU on our side.
Then trigger a long read. You will notice that the date length returned matches this 200.