efi_loader: efi_dp_split_file_path() error handling

If the path passed to efi_dp_split_file_path() does not contain a
reference to a file it returns EFI_OUT_OF_RESOURCES. This does not properly
indicate the kind of the problem that occurred. Return
EFI_INVALID_PARAMETER instead.

Update function description.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>