iio: amplifiers: ad8366: Change devm_gpiod_get() to optional and add the missed check

Since if there is no GPIO, nothing happens, replace devm_gpiod_get()
with devm_gpiod_get_optional().
Also add IS_ERR() to fix the missing-check warning.

Fixes: cee211f4 ("iio: amplifiers: ad8366: Add support for the ADA4961 DGA")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>