ASoC: Properly handle AC'97 device lifetime management

The memory that a struct device is contained in must not be freed except
from within the device's release callback. The ASoC code currently does not
adhere to this rule for the AC'97 device. This patch fixes it by moving the
freeing of the AC'97 to the release callback and splitting up the
registration and unregistration of the device into separate steps for
getting/putting the reference to the device and adding/removing it to the
device hierarchy.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>