ASoC: WM8903: Disallow all invalid gpio_cfg pdata values

The GPIO registers are 15 bits wide. Hence values, higher than 0x7fff are
not legal GPIO register values. Modify the pdata.gpio_cfg handling code
to reject all illegal values, not just WM8903_GPIO_NO_CONFIG (0x8000). This
will allow the later use of 0xffffffff as an invalid value in future device
tree bindings, meaning "don't touch this GPIO's configuration".

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>