iio: dac: ti-dac5571: fix an error code in probe()

If we have an unexpected number of channels then return -EINVAL instead
of returning success.

Fixes: df38a4a7

 ("iio: dac: add TI DAC5571 family support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210816183954.GB2068@kili
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>