libcxgb,iw_cxgb4,cxgbit: add cxgb_mk_rx_data_ack()

Add cxgb_mk_rx_data_ack() to remove duplicate
code to form CPL_RX_DATA_ACK hardware command.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>