IB/iser: Add change_queue_depth method

Add a .change_queue_depth handler to the scsi_host_template in the
iSER driver.  iscsi_change_queue_depth was added to iscsi_tcp in order
to solve the problem of queue depth which was too high for some
targets.  It is also applicable for iSER.

Signed-off-by: Erez Zilber <erezz@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>