nvme-rdma: force queue size to respect controller capability

Queue size needs to respect the Maximum Queue Entries Supported advertised by
the controller in its Capability register.

Signed-off-by: Samuel Jones <sjones@kalray.eu>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[sagig: fixed queue_size adjustment according to
Daniel Verkamp <daniel.verkamp@intel.com> comment]
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>