efi_loader: Update description of internal efi_mem_carve_out

In 74c16acc the return values where
changed, but the description was kept.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>