cxgb4vf: Read correct FL congestion threshold for T5 and T6

VF driver was reading incorrect freelist congestion notification threshold
for FLM queues when packing is enabled for T5 and T6 adapter. Fixing it
now.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>