ALSA: sound/mixart: avoid redefining {readl,write}_{le,be} accessors

If the platform already provides a definition for these accessors
do not redefine them. The warning was caught on MIPS.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>