arcmsr: limit max. number of SCSI command request

This patch limits the max. number of SCSI commmand request to avoid command
overflow.

Signed-off-by: Ching Huang <ching2048@areca.com.tw>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>