staging:iio:adc:ad7298: Use new triggered buffer setup helper function

Use the new triggered buffer setup helper function to allocate and register
buffer and pollfunc.

The previous code was not passing the temperature channel to iio_buffer_register
since the temperature channel can not be used in buffered mode. With the new
helper functions it is not possible to do this, instead the scan index for the
temperature channel is set to -1 which will cause iio_buffer_register to ignore
the channel. (Also While we are at it also assign the symbolic constant instead
of the raw value to the channel address for the temperature channel).

Also as part of the conversion drop scan_timestamp being enabled by default,
since it is a left over of an earlier cleanup.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>