tx493[89]ide: Fix length for __ide_flush_dcache_range

This fixes data corruption on PIO mode.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: stable <stable@kernel.org>
Cc: sshtylyov@ru.mvista.com
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>