cxgb4: Make sure that Freelist size is larger than Egress Congestion Threshold

We need to make sure that the Free List Size, in pointers, is at
least 2 Egress Queue Units (8 pointers/each) larger than the SGE's Egress
Congestion Threshold (in pointers).

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