ide: fix hwif_to_node()

hwif_to_node() incorrectly assumes that hwif->dev always belongs to
a PCI device.  This results in ide-cs oopsing in init_irq() after
commit c56c5648 accidentally fixed
device tree registration for ide-cs.  Fix it by using dev_to_node().

Thanks to Martin Michlmayr and Larry Finger for help with debugging
the issue.

Reported-by: Martin Michlmayr <tbm@cyrius.com>
Tested-by: Martin Michlmayr <tbm@cyrius.com>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>