ide: Fix siimage driver accessing beyond array boundary

The siimage uses an incorrect construct to access the other drive of a pair,
causing it to access beyond an array boundary on the secondary interface.

This fixes it by using the new ide_get_paired_drive() helper instead.

Bart: patch description fixes

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