[rilmodem] Set service class to 0 in call waiting query

Set service class to 0 in call waiting query as certain
operators can reject the query with specific service class

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>
Merge pull request #80 from ptakalok/master

[rilmodem] Set service class to 0 in call waiting query