x86/efistub: Avoid returning EFI_SUCCESS on error

The fail label is only used in a situation where the previous EFI API
call succeeded, and so status will be set to EFI_SUCCESS. Fix this, by
dropping the goto entirely, and call efi_exit() with the correct error
code.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>