staging:iio:ad2s90: Check channel type at read_raw

This patch adds a channel type check at the beginning of the
ad2s90_read_raw function. Since ad2s90 has only one channel, it just
checks if the given channel is the expected one and if not, return
-EINVAL.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>