IB/iser: set can_queue earlier to allow setting higher queue depth

We need to set can_queue earlier than when enabling the scsi host.
in a blk-mq enabled environment, the tagset allocation is taken
from can_queue which cannot be modified later. Also, pass an updated
.can_queue to iscsi_session_setup to have enough iscsi tasks allocated
in the session kfifo.

Reported-by: Karandeep Chahal <karandeepchahal@gmail.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>