iio: inkern: fix return value in devm_of_iio_channel_get_by_name()

iio: inkern: fix return value in devm_of_iio_channel_get_by_name()

of_iio_channel_get_by_name() can either return NULL or an error pointer
so that only doing IS_ERR() is not enough. Fix it by checking the NULL
pointer case and return -ENODEV in that case. Note this is done like this
so that users of the function (which only check for error pointers) do
not need to be changed. This is not ideal since we are losing error codes
and as such, in a follow up change, things will be unified so that
of_iio_channel_get_by_name() only returns error codes.

Fixes: 66395f5e113d ("iio: provide of_iio_channel_get_by_name() and devm_ version it")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220715122903.332535-3-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>