Originally the FPGA ID checking switch statement left disabled all the
TS implemented FPGA devices if the ID was unknown to it.  Michael Spang
created a fix in f9b1184e

 that changed the default to enable the devices
in the event TS silently release more revisions into the wild, this
unfortunately breaks custom FPGA bitstreams.

This patch amends the switch statement so that the TS devices are only
enabled if on the revision number is unknown (whilst the magic matches).

Changelog:
 v2: neater implementation and some cosmetic changes
 v1: initial release <20110305112937.GA22117@chipmunk>

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>