ide-disk: merge LBA28 and LBA48 Host Protected Area support code (take 2)

* Merge idedisk_{read_native,set}_max_address_ext() into
  idedisk_{read_native,set}_max_address().

v2:
* Remove LBA48 code leftover from idedisk_read_native_max_address()
  ('high' variable initialization).  (Noticed by Sergei).

There should be no functionality changes caused by this patch.

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