Fix compilation on arm64 with FP16 when disabled

If building with -mcpu=native or any other setting which implies the current
CPU has FP16 but with intrinsics disabled, we mistakenly try to use it even
though convolution.hpp conditionally defines it correctly based on whether
we should *use it*. convolution.cpp on the other hand was mismatched and
trying to use it if the CPU supported it, even if not enabled in the build
system.

Make the guards match.

Bug: https://bugs.gentoo.org/913031
Signed-off-by: Sam James <sam@gentoo.org>

modules/dnn/src/layers/cpu_kernels/convolution.hpp

@@ -14,7 +14,7 @@
 #define CONV_NR_FP32 28
 
 // The FP16 can only be supported by ARM64 and with FP16 FMA supported.
-#ifdef __ARM_FEATURE_FP16_VECTOR_ARITHMETIC // check FP16 FMA.
+#if defined(__ARM_FEATURE_FP16_VECTOR_ARITHMETIC) && CV_FP16 // check FP16 FMA.
 #define CONV_ARM_FP16 1
 #endif