iio/adc: depend on SYSFS instead of selecting it

Drivers should not 'select' a subsystem. Instead they should depend
on it. If the subsystem is disabled, the user probably did that for
a purpose and one driver shouldn't be changing that.

This also makes all IIO drivers consistent w.r.t depending on SYSFS
instead of selecting it.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>