scsi: arcmsr: Fix doorbell status being updated late on ARC-1886

It is possible for the IOP to be delayed in updating the doorbell
status. The doorbell status should not be 0 so loop until the value
changes.

Link: https://lore.kernel.org/r/afdfdf7eabecf14632492c4987a6b9ac6312a7ad.camel@areca.com.tw


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