ALSA: ppc: keywest: drop using attach adapter

As we now have deferred probing, we can use a custom mechanism and
finally get rid of this legacy interface from the i2c core.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Tested-by: Dan DeVoto <dand1972@yahoo.com>
Tested-by: Mark Elliott <txlitebeer@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>