libcxgb, iw_cxgb4, cxgbit: add cxgb_mk_close_con_req()

Add cxgb_mk_close_con_req() to remove duplicate
code to form CPL_CLOSE_CON_REQ hardware command.

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