efi_loader: correctly determine length of empty device path

efi_dp_size() is meant to return the device path length without the end
node.

The length of a device path containing only an end node was incorrectly
reported as 4.

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