scsi: pm8001: Simplify pm8001_task_exec()

The main part of the pm8001_task_exec() function uses a do {} while(0) loop
that is useless and only makes the code harder to read. Remove this
loop. The unnecessary local variable t is also removed.

Additionally, avoid repeatedly declaring "struct task_status_struct *ts" to
handle error cases by declaring this variable for the entire function
scope. This allows simplifying the error cases, and together with the
addition of blank lines make the code more readable.

Finally, handling of the running_req counter is fixed to avoid decrementing
it without a corresponding incrementation in the case of an invalid task
protocol.

Link: https://lore.kernel.org/r/20220220031810.738362-29-damien.lemoal@opensource.wdc.com
Reviewed-by: John Garry <john.garry@huawei.com>
Reviewed-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>