res_rtp_asterisk.c: Correct coefficient in MOS calculation.

res_rtp_asterisk.c: Correct coefficient in MOS calculation.

Media Experience Score relies on incorrect pseudo_mos variable
calculation. According to forming an opinion section of the
documentation, calculation relies on ITU-T G.107 standard:

    https://docs.asterisk.org/Deployment/Media-Experience-Score/#forming-an-opinion

ITU-T G.107 Annex B suggests to calculate MOS with a coefficient
"seven times ten to the power of negative six", 7 * 10^(-6). which
would mean 6 digits after the decimal point. Current implementation
has 7 digits after the decimal point, which downrates the calls.

Fixes: #597