mips au1xxx_ide.h: use NULL as firmware-revision wildcard

This updates the DMA whitelist in MIPS specific au1xxx ide
driver to use NULL instead of "ALL" as the wildcard.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Dave Jones <davej@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>