cxgb4i: Increased the value of MAX_IMM_TX_PKT_LEN from 128 to 256 bytes

This helps improving the latency of small packets.

Signed-off-by: Rakesh Ranjan <rakesh@chelsio.com>
Signed-off-by: Karen Xie <kxie@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>