core: fix FP16 conversion with CV_DISABLE_OPTIMIZATION option

Reproducer:
    cmake -DCPU_BASELINE=AVX2 -DCV_DISABLE_OPTIMIZATION=ON ...

cmake/OpenCVCompilerOptimizations.cmake

@@ -703,16 +703,19 @@ macro(ocv_compiler_optimization_fill_cpu_config)
       set(OPENCV_CPU_CONTROL_DEFINITIONS_CONFIGMAKE "${OPENCV_CPU_CONTROL_DEFINITIONS_CONFIGMAKE}
 #if !defined CV_DISABLE_OPTIMIZATION && defined CV_ENABLE_INTRINSICS && defined CV_CPU_COMPILE_${OPT}
 #  define CV_TRY_${OPT} 1
+#  define CV_CPU_FORCE_${OPT} 1
 #  define CV_CPU_HAS_SUPPORT_${OPT} 1
 #  define CV_CPU_CALL_${OPT}(fn, args) return (cpu_baseline::fn args)
 #  define CV_CPU_CALL_${OPT}_(fn, args) return (opt_${OPT}::fn args)
 #elif !defined CV_DISABLE_OPTIMIZATION && defined CV_ENABLE_INTRINSICS && defined CV_CPU_DISPATCH_COMPILE_${OPT}
 #  define CV_TRY_${OPT} 1
+#  define CV_CPU_FORCE_${OPT} 0
 #  define CV_CPU_HAS_SUPPORT_${OPT} (cv::checkHardwareSupport(CV_CPU_${OPT}))
 #  define CV_CPU_CALL_${OPT}(fn, args) if (CV_CPU_HAS_SUPPORT_${OPT}) return (opt_${OPT}::fn args)
 #  define CV_CPU_CALL_${OPT}_(fn, args) if (CV_CPU_HAS_SUPPORT_${OPT}) return (opt_${OPT}::fn args)
 #else
 #  define CV_TRY_${OPT} 0
+#  define CV_CPU_FORCE_${OPT} 0
 #  define CV_CPU_HAS_SUPPORT_${OPT} 0
 #  define CV_CPU_CALL_${OPT}(fn, args)
 #  define CV_CPU_CALL_${OPT}_(fn, args)

modules/core/src/convert.cpp

@@ -1363,7 +1363,7 @@ cvtScaleHalf_<float, short>( const float* src, size_t sstep, short* dst, size_t
 {
     CV_CPU_CALL_FP16_(cvtScaleHalf_SIMD32f16f, (src, sstep, dst, dstep, size));
 
-#if !defined(CV_CPU_COMPILE_FP16)
+#if !CV_CPU_FORCE_FP16
     sstep /= sizeof(src[0]);
     dstep /= sizeof(dst[0]);
 
@@ -1382,7 +1382,7 @@ cvtScaleHalf_<short, float>( const short* src, size_t sstep, float* dst, size_t
 {
     CV_CPU_CALL_FP16_(cvtScaleHalf_SIMD16f32f, (src, sstep, dst, dstep, size));
 
-#if !defined(CV_CPU_COMPILE_FP16)
+#if !CV_CPU_FORCE_FP16
     sstep /= sizeof(src[0]);
     dstep /= sizeof(dst[0]);