cxgb4 : Fix bug in DCB app deletion

Unlike CEE, IEEE has a bespoke app delete call and does not rely on priority
for app deletion

Fixes : 2376c879

 ('cxgb4 : Improve handling of DCB negotiation or loss
 thereof')

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>