ASoC: wm8904: Don't touch GPIO configs set to 0xFFFF

ASoC: wm8904: Don't touch GPIO configs set to 0xFFFF

When updating the GPIO registers, do nothing for all fields of gpio_cfg
that are "0xFFFF".

This "do nothing" flag used to be 0 to easily check whether the gpio_cfg
field was actually set inside pdata or left empty (default).

However, 0 is a valid configuration for these registers, while 0xFFFF is
not.

With this change, users can explicitly set them to 0.
Not setting gpio_cfg in the platform data will now lead to setting all
GPIO registers to 0 instead of leaving them unset.

No one is using this platform data with this codec.

The change gets the driver ready to properly set gpio_cfg from the DT.

Datasheet: https://statics.cirrus.com/pubs/proDatasheet/WM8904_Rev4.1.pdf
Signed-off-by: Ernest Van Hoecke <ernest.vanhoecke@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://patch.msgid.link/20250319142059.46692-3-francesco@dolcini.it
Signed-off-by: Mark Brown <broonie@kernel.org>