scsi: arcmsr: Fix reading empty buffer length

Fix empty buffer length error which causes ARECA CLI app command timeout.

Signed-off-by: ching Huang <ching2048@areca.com.tw>
Link: https://lore.kernel.org/r/4aec4385abc6eab10b575c28130ec615a144492d.camel@areca.com.tw
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>