i386/tdx: fix uninitialized variable warning in tdx_check_features

tdx_fetch_cpuid() only sets the output ret parameter on the error
path. GCC cannot prove that r is always initialized before use in
the caller, triggering -Werror=maybe-uninitialized.

Initialize r to -1 to silence the warning.

Fixes: 228e40f33048 ("i386/tdx: Fetch and validate CPUID of TD guest")
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Signed-off-by: Marc-Andre Lureau <marcandre.lureau@redhat.com>