efi/libstub/x86: Fix EFI server boot failure

efi/libstub/x86: Fix EFI server boot failure

x86_64 EFI systems are unable to boot due to a typo in a recent commit:

  EFI config tables not found.
   -- System halted

This was probably due to the absense of CONFIG_EFI_MIXED=y in testing.

Fixes: e38d6bdd0317 ("efi/libstub/x86: Use const attribute for efi_is_64bit()")
Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: tglx@linutronix.de
Cc: linux-efi@vger.kernel.org
Link: https://lore.kernel.org/r/20200122191430.4888-1-cai@lca.pw