efi_loader: efi_dp_from_lo() should skip VenMedia node

The 'efidebug boot dump' command should not display the VenMedia() device
path node preceding the device path of the initial ram disk.

By letting efi_dp_from_lo() skip the VenMedia() device path node we can
simplify the coding.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>