RDMA/i40iw: Correct STag mask to min of 14 bits

STag index mask is calculated incorrectly, missing
the 14 bits minimum requirement. Add max macro to use
either # of MRs or 14 bits in the mask size calculation.

Signed-off-by: Tatyana Nikolova <Tatyana.E.Nikolova@intel.com>
Signed-off-by: Faisal Latif <faisal.latif@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>