cpu: conv: x64: use proper offset when reading args from stack

This is a fixup for 778121a:
cpu: conv: x64: Implement bwd_w algorithm targeting nxc data formats

Author: Fomenko, Evarist M

src/cpu/x64/jit_avx512_common_conv_kernel.cpp

@@ -5193,7 +5193,7 @@ void jit_avx512_common_conv_bwd_weights_kernel_f32::generate_microkernel() {
 #ifdef _WIN32
     // Must be a scratch register since load is before preamble
     reg64_t reg_owb = rax;
-    mov(reg_owb, ptr[rsp + 8]);
+    mov(reg_owb, ptr[get_stack_params_address(false)]);
 #else
     reg64_t reg_owb = abi_param5;
 #endif