efi_loader: comments for EFI_DEVICE_PATH_TO_TEXT_PROTOCOL

Provide comments for efi_convert_device_node_to_text()
and efi_convert_device_path_to_text().

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