CSFB SS: add setCallWaiting API for service class

When calling setCallWaiting by CSFB MMI code,
SERVICE_CLASS_VOICE is always used as service class.
But it is not intended.
Service class should be set as received
regarding CSFB MMI code.

Bug: 146531989

Change-Id: I85b93990d68fc42a8a8a80977ab8d13ebfd25708
Signed-off-by: Jang Hayeong <hayeong.jang@samsung.com>