net/i40e: fix clang build with 16B descriptors

net/i40e: fix clang build with 16B descriptors

When compiling with 16B descriptor support enabled, clang compiles gave
an error, complaining that the final parameter of _mm256_blend_epi32()
had to be an immediate value (i.e. compile-time constant):

 i40e_rxtx_vec_avx2.c:561:21: error: argument to
'__builtin_ia32_pblendd256' must be a constant integer
   __m256i tmp0_1 = _mm256_blend_epi32(fdir_zero_mask,
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

While it appears that GCC was able to convert the constant variable
value "fdir_blend_mask" into the blend call, clang was not doing so. To
guarantee the use of an immediate we convert the variable value to a
"#define".

Fixes: 7d087a0a8b8e ("net/i40e: support flow director on AVX Rx")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Xiaolong Ye <xiaolong.ye@intel.com>