staging:iio:adc:ad7793 use iio_sw_buffer_preenable to avoid repitition.

Now the generic function caches the scan size a lot of what was here
was redundant and is removed.

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>