i40iw: Send last streaming mode message for loopback connections

Send a zero length last streaming mode message for loopback
connections to synchronize between accepting QP and connecting QP.
This avoids data transfer to start on the accepting QP before
the connecting QP is in RTS. Also remove function i40iw_loopback_nop()
as it is no longer used.

Fixes: f27b4746 ("i40iw: add connection management code")

Signed-off-by: Tatyana Nikolova <tatyana.e.nikolova@intel.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>