iio: adc: ad7476: remove redundant null check on an array

The null check on st->chip_info->convst_channel is redundant because
convst_channel is a 2 element array of struct iio_chan_spec objects
and this can never be null. Fix this by removing the null check.

Addresses-Coverity: ("Array compared against 0")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>