ASoC: Fix WM8753 DAIs unregistering

WM8753 uses a tricky way to switch DAIs "on the fly", for that it
registers 2 dummy DAIs and substitutes them depending on mixer control.

List element of registered dummy DAIs should be preserved to allow
unregistering of DAIs on module unload.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>