qed: Add DCBX API - qed_dcbx_get_priority_tc()

The API receives a priority and looks for the TC it is mapped to in the
operational DCBX configuration. The API returns QED_DCBX_DEFAULT_TC (0)
when DCBX is disabled.

Signed-off-by: Michal Kalderon <michal.kalderon@cavium.com>
Signed-off-by: Ariel Elior <ariel.elior@cavium.com>
Signed-off-by: Denis Bolotin <denis.bolotin@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>