iio: adc: pac1921: add missing error return in probe()

This error path was intended to return, and not just print an error.  The
current code will lead to an error pointer dereference.

Fixes: 371f778b ("iio: adc: add support for pac1921")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Matteo Martelli <matteomartelli3@gmail.com>
Link: https://patch.msgid.link/1fa4ab12-0939-477d-bc92-306fd32e4fd9@stanley.mountain


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>