IB/core: Export a common fw_ver sysfs entry

Now that all the devices have stopped exporting their own sysfs
entry points we can have the core export this on their behalf.

Eventually this may be removed but this provides for backwards
compatibility.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>