[rilmodem] Change call forwarding default service class per RIL design

RIL expects service class BEARER_CLASS_VOICE in
RIL_REQUEST_SET_CALL_FORWARD and SERVICE_CLASS_NONE in
RIL_REQUEST_QUERY_CALL_FORWARD_STATUS.

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>
Merge pull request #186 from ptakalok/call_forw

[rilmodem] Change call forwarding default service class per RIL design