efi_loader: correct device path check

Since commit 226cddbe ("efi_loader: check device path in
InstallMultipleProtocolInterfaces") iPXE fails to access the network.

LocateDevicePath() returns EFI_SUCCESS even if a shorter path is found as a
partial match. It returns the remaining path. So to be sure that we found a
complete match we need to check that the remaining path refers to an end
node.

Provide debug output if a device path has already been installed.

Fixes: 226cddbe ("efi_loader: check device path in
       InstallMultipleProtocolInterfaces")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>