iio: ad9523: Fix return value for ad952x_store()

A sysfs write callback function needs to either return the number of
consumed characters or an error.

The ad952x_store() function currently returns 0 if the input value was "0",
this will signal that no characters have been consumed and the function
will be called repeatedly in a loop indefinitely. Fix this by returning
number of supplied characters to indicate that the whole input string has
been consumed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Fixes: cd1678f9 ("iio: frequency: New driver for AD9523 SPI Low Jitter Clock Generator")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>