Improve ADSL connection speeds by tweaking SNR threshold.

Improve ADSL connection speed on RTA1320v6:

> telnet 10.0.0.1 
Trying 10.0.0.1...
Connected to rta1320.home.
Escape character is '^]'.
Dynalink ADSL2+ Router RTA1320 REV2 16M NZ 
Software Version: 3.65p
Login name: admin
Password: 

My usual connection speed out of the box isn't that great:

> adsl info
adsl: ADSL driver and PHY status
Status: ShowtimeRetrain Reason: 8000
  Channel: FAST, Upstream rate = 961 Kbps, Downstream rate = 6054 Kbps

Lower the SNR threshold:

> adsl configure --snr 1

After a few moments, the device retrains and attains a higher speed:

> adsl info
adsl: ADSL driver and PHY status
Status: ShowtimeRetrain Reason: 8000
  Channel: FAST, Upstream rate = 969 Kbps, Downstream rate = 7991 Kbps

We can improve the speed even more by lowering the SNR. We can't enter negative numbers, but the value itself is unsigned 16-bits, so a large positive number has the same effect:

> adsl configure --snr 65460
> adsl info
adsl: ADSL driver and PHY status
Status: ShowtimeRetrain Reason: 8000
  Channel: FAST, Upstream rate = 973 Kbps, Downstream rate = 10089 Kbps

Ultimately, the lower you make the SNR threshold, the more chance you have of errors or disconnects. Trial and error is the best option here.