softap: sscanf sanity check for SoftAP service

The sscanf return value is verified whether the three channel
variables, i.e. start channel, end channel and channel band,
are properly updated. If malformed input is given to sscanf, then
these variables may not properly updated. To provide validation
of input,  verifying that sscanf returned a correct value of 3.

Change-Id: Ibd185e7a0ffcf5803927990531fc33f0e8977cc9
CRs-fixed: 554349