Lantiq: make possible to tweak DSL SRN from UCI

Lantiq: make possible to tweak DSL SRN from UCI

This patch makes possible to tweak the downstream SNR margin on
Lantiq DSL devices.

The UCI parameter 'network.dsl.ds_snr_offset' is used to set the SNR
margin offset. It accepts values in range -50 to +50 in 0.1 dB units.

The SNR margin can thus be modified in range -5.0 to +5.0 dB in 0.1 dB
steps.

Currently this should only affect ADSL (not VDSL). It should be very
easy to make this work also on VDSL lines, but since I couldn't test
on VDSL lines this patch does not do that yet.

I have also a patch for LUCI about this, that I could submit.

Tested on FB3370 (Lantiq VR9) and Telecom Italia ADSL2+ line.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>