ALSA: ASoC: Check for exact register match in wm97xx_reset()

To provide added robustness in case an AC97 controller reads back all
zeros in error cases check for an exact match when testing to see if
resets have brought the codec back.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>