scsi: qla2xxx: Increase the size of the mailbox arrays from 4 to 8

This patch avoids that Coverity complains that qla2x00_async_event() writes
outside the bounds of the mb[] arrays (MBA_IDC_AEN case).

Cc: Himanshu Madhani <hmadhani@marvell.com>
Cc: Giridhar Malavali <gmalavali@marvell.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>