staging: iio: cdc: fix improper return value

At the end of function ad7150_write_event_config(), directly returns 0.
As a result, the errors will be ignored by the callers. It may be better
to return variable "ret".

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>