IB/srp: Increase supported CDB size

Set the Scsi_Host's max_cmd_len from 12 (default) to 16 for
SRP. Otherwise scsi_dispatch_cmd() won't pass down certain commands
such as READ CAPACITY 16, required for supporting disks > 2TB.

Signed-off-by: Arne Redlich <arne.redlich@xiranet.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>