[rilmodem] Check and set tech prefence as configured

After flashing modem uses default technology preference of it's
own. This fix forces it to 3G/2g only, if configuration is set
so.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #100 from jkangas/master

Request for pulling technology preference checking and setting at startup to main branch