iw_cxgb4: correctly enforce the max reg_mr depth

The code was mistakenly using the length of the page array memory instead
of the depth of the page array.

This would cause MR creation to fail in some cases.

Fixes: 8376b86d ("iw_cxgb4: Support the new memory registration API")
Cc: stable@vger.kernel.org
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>