efi_loader: do_bootefi_exec should always return an EFI status code

The return type of do_bootefi_exec() is efi_status_t. So in case
of an error we should always return an EFI status code.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>