staging:iio:adc:ad799x use iio_sw_buffer_preenable to avoid code repitiion

Using this generic function cuts down on repeated code at the cost of
a little overhead in a slow path.

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