iio: light: cm32181: Clean up the probe function a bit

3 small cleanups to cm32181_probe():

1. Do not log an error when we fail to allocate memory (as a general
rule drivers do not log errors for this as the kernel will already
have complained loudly that it could not alloc the mem).

2. Remove the i2c_set_clientdata() call, we never use i2c_get_clientdata()
or dev_get_drvdata() anywhere.

3. Add a dev helper variable and use it in various places instead of
&client->dev.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>