ide: add ide_busy_sleep() helper

Add ide_busy_sleep() helper and use it in do_probe(),
enable_nest() and probe_hwif().

As a nice side-effect this fixes a minor bug in do_probe()
(the code was reading status register without any delay).

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>