ide-pmac: use __ide_wait_stat()

* Use __ide_wait_stat() instead of wait_for_ready() in pmac_ide_do_setfeature().

While at it do following changes to match __ide_wait_stat() call in
ide_config_drive_speed():

* Wait WAIT_CMD time (20 sec) instead of 2 sec for device to clear BUSY_STAT.

* Check DRQ_STAT bit (shouldn't be set for good device status).

Also remove no longer needed wait_for_ready() from ide-iops.c.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>