iio: adc: ad7923: Fix checkpatch warning

Fix checkpatch warning:
WARNING: Prefer 'unsigned int' to bare use of 'unsigned'
+static int ad7923_scan_direct(struct ad7923_state *st, unsigned ch)

Signed-off-by: Daniel Junho <djunho@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>