hw/isa/isa-bus: Set category of the "isabus-bridge" device

It has "bridge" in its name, so it should be in the category
DEVICE_CATEGORY_BRIDGE.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>