iio: light: veml6030: use read_avail() for available attributes

iio: light: veml6030: use read_avail() for available attributes

Drop custom attributes by using the standard read_avail() callback to
read scale and integration time. When at it, add the integration time
and scale attributes fro the WHITE channel, as they modify its value as
well. To avoid breaking the current ABI, these attributes must be kept
as separate for both channels even though they are shared under the
hood.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20241001-veml6035-v3-5-d789f6ff147c@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>