cxgb4: Increased the value of MAX_IMM_TX_PKT_LEN from 128 to 256 bytes

This allows a significant latency drop for packets of sizes between 128 and 192
bytes

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