ASoC: Tegra: Don't store snd_soc_jack_gpio in an array

Storing the struct in an array makes the assignments to the GPIO member a
little non-obvious, and is pointless when there's only a single GPIO.

(I thought I fixed this during the review cycle when first submitting this
driver, but I guess I overlooked that)

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>