iio: at91: fix error return code in at91_adc_probe()

Fix to return -ENODEV instead of 0 if non-TSMR adc don't
support, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>