dcb: Add DCBX capabilities bitmask to the get_ieee response

Adding the capabilities bitmask to the get_ieee response allows
user space to determine the current DCBX mode. Either CEE or IEEE
this is useful with devices that support switching between modes
where knowing the current state is relevant.

Derived from work by Mark Rustad

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>