efi_loader: fix efi_convert_device_node_to_text

We need to implement to different functions for the
EFI_DEVICE_PATH_TO_TEXT_PROTOCOL:
ConvertDeviceNodeToText
ConvertDevicePathToText

A recent patch screwed up efi_convert_device_node_to_text
to expect a device path and not a node.

The patch makes both service functions work again.

efi_convert_device_node_to_text is renamed to
efi_convert_single_device_node_to_text and
efi_convert_device_node_to_text_ext is renamed to
efi_convert_device_node_to_text to avoid future
confusion.

A test of ConvertDeviceNodeToText will be provided in
a follow-up patch.

Fixes: adae4313 efi_loader: flesh out device-path to text
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>