staging:iio:adc:ad799x: Use new triggered buffer setup helper function

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

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>