IB/rdamvt: Fix rdmavt s_ack_queue sizing

rdmavt allows the driver to specify the size of the ack queue, but
only uses it for the modify QP limit testing for setting the atomic
limit value.

The driver dependent size is now used to size the s_ack_queue ring
dynamicially.

Since the driver knows its size, the driver will use its define
for any ring size dependent code.

Reviewed-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>