set proper default values for puSch10xSnr and puCch10xSnr

20 was wrong, it means 2 dB.

200 is a bit high on my setup. Today the code hardcodes the noise
level as 30dB and a target of 20dB above noise is chosen by default.
Some more work is needed to estimate the real noise level. Some
more work is needed to set proper defaults.