iio: adc: qcom-spmi-adc5: Tidy up adc5_get_fw_data() error messages

iio: adc: qcom-spmi-adc5: Tidy up adc5_get_fw_data() error messages

In the event that no channels (child nodes) are defined, the adc5 driver
will provide a generic error message indicating that adc5_get_fw_data()
returned -EINVAL. In all other error cases we get two error messages,
one helpful and the generic one.

Add a specific error message for the no channels case, and drop the
generic one, in order to improve the generates log prints in both cases.

Signed-off-by: Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>
Link: https://patch.msgid.link/20241004-spmi-adc5-no-channel-error-v1-1-1a43d13ae967@oss.qualcomm.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>