i40iw: Zero-out consumer key on allocate stag for FMR

If the application invalidates the MR before the FMR WR, HW parses the
consumer key portion of the stag and returns an invalid stag key
Asynchronous Event (AE) that tears down the QP.

Fix this by zeroing-out the consumer key portion of the allocated stag
returned to application for FMR.

Fixes: ee855d3b93f3 ("RDMA/i40iw: Add base memory management extensions")
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>