iio: adc: ad7606: Disable PWM usage for non backend version

Since the pwm was introduced before backend, there was a mock use, with
a GPIO emulation. Now that iio backend is introduced, the mock use can
be removed.

Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20241015-ad7606_add_iio_backend_support-v5-8-654faf1ae08c@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>