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: 796eb8d2 ("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

arch/x86/boot/compressed/eboot.c

@@ -32,7 +32,7 @@ __attribute_const__ bool efi_is_64bit(void)
 {
 	if (IS_ENABLED(CONFIG_EFI_MIXED))
 		return efi_is64;
-	return IS_ENABLED(CONFIG_X64_64);
+	return IS_ENABLED(CONFIG_X86_64);
 }
 
 static efi_status_t