IB/srpt: Limit the send and receive queue sizes to what the HCA supports

Additionally, correct the comment about ch->rq_size.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>