Fix signal quality and SNR statistics in linuxdvb input

The signal quality and SNR report in a fully ranged u16 and 0.1 dB resolution respectively. I switched to the master branch recently, and I had the same issue in the 3.4 release. Now that the dvb-rewrite is in, I propose these two very minor changes.

Tested with a DRKX demodulator (a PCTV 510e DVB-C setup, kernel 3.11.6). I also cross-referenced it with the dvbv5-tools and the dvbv5 API. There are at least a dozen other frontends using the same units, so I hope that this is correct for all of them.