scsi: hisi_sas: tidy channel interrupt handler for v3 hw

The ISR of channel interrupt of v3 hw is a little long and messy. This
patch tidies it by relocating CHL_INT1 and CHL_INT2 handling to new
function separately.

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>