efi_loader: correct find simple file system protocol

In contrast to the description the code did not split the device
path into device part and file part.

The code should use the installed protocol and not refer to the
internal structure of the the disk object.

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