libcxgb,iw_cxgb4,cxgbit: add cxgb_mk_tid_release()

Add cxgb_mk_tid_release() to remove duplicate code
to form CPL_TID_RELEASE hardware command.

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