Turn 'adc128_probe()' into a full resource managed function to simplify the
code.

This way, the .remove function can be removed.
Doing so, the only 'spi_get_drvdata()' call is removed and the
corresponding 'spi_set_drvdata()' can be removed as well.
Suggested-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/4fa7fcc59c40e27af0569138d656c698a53dbd44.1630002770.git.christophe.jaillet@wanadoo.frSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>