ide-scsi: fix Interrupt Reason checking in idescsi_pc_intr()

Set PC_FLAG_WRITING pc flag in idescsi_queue() (if needed)
and then fix Interrupt Reason checking in idescsi_pc_intr().

Cc: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>