ide: improve handling of Power Management requests

Make hwif->rq point to PM request during PM sequence and do not allow
any other types of requests to slip in (the old comment was never correct
as there should be no such requests generated during PM sequence).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>