efi_loader: do not use unsupported printf code

Using %zu for efi_intn_t (ssize_t) creates a build warning. Anyway %zu is
not supported by efi_st_error(). So let's convert to int.

Our implementation of StriColl() only returns -1, 0, or 1.

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