iw_cxgb4: invalidate the mr when posting a read_w_inv wr

Also, rearrange things a bit to have a common c4iw_invalidate_mr()
function used everywhere that we need to invalidate.

Fixes: 49b53a93 ("iw_cxgb4: add fast-path for small REG_MR operations")
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>