scsi: hisi_sas: fix return value of hisi_sas_task_prep()

It is an implicit regulation that error code that function returned should
be negative. But hisi_sas_task_prep() doesn't follow this.  This may cause
problems in the upper layer code.

For example, in sas_expander.c of libsas, smp_execute_task_sg() may return
the number of bytes of underrun. It will be conflicted with the scenaio
lldd_execute_task() return an positive error code.

This patch change the return value from SAS_PHY_DOWN to -ECOMM in
hisi_sas_task_prep().

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>