iw_cxgb4: allocate wait object for each memory object

Remove the local stack allocated c4iw_wr_wait object in preparation for
correctly handling timeouts.

Also refactored some code to simplify it and make errpath unwinding
more readable.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>