hwmon: (adcxx) Remove redundant spi_set_drvdata

Driver core sets driver data to NULL upon failure or remove.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Marc Pignat <marc.pignat@hevs.ch>
Acked-by: Marc Pignat <marc.pignat@hevs.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>