x86/tdx: Fix typo in comment in __tdx_hypercall()

Comment in __tdx_hypercall() points that RAX==0 indicates TDVMCALL
failure which is opposite of the truth: RAX==0 is success.

Fix the comment. No functional changes.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230126221159.8635-2-kirill.shutemov%40linux.intel.com