iio: adc: ti_am335x_adc: avoid double free of buffer.

The driver is missing the iio_buffer_attach() call. As such it will attempt
to free the buffer twice on removal.

Introduced in commit 9e69c9 ("iio: Add reference counting for buffers").

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>