scsi: arcmsr: replace constant ARCMSR_MAX_FREECCB_NUM

Replace constant ARCMSR_MAX_FREECCB_NUM by variable acb->maxFreeCCB that
was received from firmware.

Signed-off-by: Ching Huang <ching2048@areca.com.tw>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>