i40iw: Do not allow posting WR after QP is flushed

A Work Request (WR) posted after QP is flushed will not
get a flush completion.

Correct this problem by not allowing posting of WRs
after a QP is flushed.

Fixes: d3749841 ("i40iw: add files for iwarp interface")
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>