ASoC: Support AC97 link off by default on WM9712

The WM9712 can be configured by resistor strapping GPIO4 to behave like
the WM9713 and default to leaving the AC97 link disabled after cold
reset until a warm reset occurs.  In this configuration we need to issue
a warm reset after cold to bring the link up so do so.  The warm reset
will be harmless on systems that don't need it.

[Changelog rewritten to document the reasoning. -- broonie]

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>