csp_thread: windows: Replace thread_create in usart_windows.c
usart_windows.c is already Windows specific usart driver.  Replace the
wrapper function with native _beginthreadex().
@@ -142,8 +142,8 @@ int csp_usart_open(const csp_usart_conf_t * conf, csp_usart_callback_t rx_callba
ctx->fd = fd;
ctx->isListening = 1;
- res = csp_windows_thread_create(usart_rx_thread, "usart_rx", 0, ctx, 0, &ctx->rx_thread);
- if (res) {
+ uintptr_t ret = _beginthreadex(NULL, 0, usart_rx_thread, NULL, 0, NULL);
+ if (ret == 0) {
CloseHandle(ctx->fd);
free(ctx);
return res;
