Fix a bug of convertTo.

The bug was found that all 3-channel oclMat's were converted to
4-channel oclMat's after using convertTo function.

modules/ocl/src/matrix_operations.cpp

@@ -394,7 +394,7 @@ void cv::ocl::oclMat::convertTo( oclMat &dst, int rtype, double alpha, double be
     if( rtype < 0 )
         rtype = type();
     else
-        rtype = CV_MAKETYPE(CV_MAT_DEPTH(rtype), oclchannels());
+        rtype = CV_MAKETYPE(CV_MAT_DEPTH(rtype), channels());
 
     //int scn = channels();
     int sdepth = depth(), ddepth = CV_MAT_DEPTH(rtype);