scsi: arcmsr: Ensure getting a free ccb is done under the spin_lock

Ensure getting a free ccb is done under the spin_lock.

Link: https://lore.kernel.org/r/f3ec374cada762149911ff35e92a68a15a7d0327.camel@areca.com.tw


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