ide: add __ide_wait_stat() helper

* Split off checking of the status register from ide_wait_stat() to
  __ide_wait_stat() helper.

* Use the new helper in ide_config_drive_speed().  The only change in the
  functionality is that the function now fails if after 20 sec (WAIT_CMD)
  device is still busy (BUSY_STAT bit is set) while previously instead of
  failing the function continued with checking for the correct device status
  (which would give the device additional 10 usec to clear BUSY_STAT bit).

* Remove stale comment for ide_config_drive_speed().

* Remove duplicate comment for ide_wait_stat() from <linux/ide.h>.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>