ALSA: ppc: keywest: Don't use i2c_client->driver

The 'driver' field of the i2c_client struct is redundant and is going to be
removed. Use 'to_i2c_driver(client->dev.driver)' instead to get direct
access to the i2c_driver struct.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>