Weaken breaks/replaces of libsane-hpaio against hplip to ease backporting

When creating a backport of hplip (say with version 3.17.10+repack0-5~bpo9+1) the resulting packages for hplip and libsane-hpaio cannot be installed both because hplip's version is smaller than 3.17.10+repack0-5 and so conflicts with libsane-hpaio.

This fix this the Breaks:/Replaces: relation is slightly weakened to make the result installable.