staging:iio:ad2s90: Add comment to device state mutex

Fix the checkpatch.pl issue:
"CHECK: struct mutex definition without comment".

Signed-off-by: Victor Colombo <victorcolombo@gmail.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>