iio: dac: ad3552r-hs: fix uninitialized data ni ad3552r_hs_write_data_source()

iio: dac: ad3552r-hs: fix uninitialized data ni ad3552r_hs_write_data_source()

If the *ppos value is non-zero then the simple_write_to_buffer() function
won't initialized the start of the buf[] buffer.  Non-zero values for
*ppos won't work here generally, so just test for them and return -ENOSPC
at the start of the function.

Fixes: b1c5d68ea66e ("iio: dac: ad3552r-hs: add support for internal ramp")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Angelo Dureghello <adureghello@baylibre.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>