Improve eisa bus probe by deciding the number of logical eisa slots from

Improve eisa bus probe by deciding the number of logical eisa slots from
the motherboard eisa id, instead of using a ``one size fits all'' value which
is too large on more than half the eisa-capable alpha designs.

The id -> slot # logic is based on the alpha ECU configuration files, so we
should not perform worse than ECU itself (and see all slots ECU sees too).