IB/core: Add get FW version string to the core

Allow for a common core function to get firmware version strings
from the individual devices.

In later patches this format can then then be used to pass a
properly formated version string through the IPoIB layer.

The problem with the current code in the IPoIB layer is that it is
specific to certain hardware types.

Furthermore, this gives us a common function through which the core
can provide a common sysfs entry.  Eventually we may want to
remove the sysfs export but this provides for user space 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>