scsi: hisi_sas: service v2 hw CQ ISR with tasklet

Currently the all the slot processing for the completion queue is done
in ISR context. It is judged that the slot processing can take a long
time, especially when a SATA NCQ completes (upto 32 slots).

So, as a solution, defer the bulk of the ISR processing to tasklet
context. Each CQ will have its down tasklet.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Xiang Chen <chenxiang66@hisilicon.com>
Reviewed-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>