iio: add missing MODULE_DESCRIPTION() macros

iio: add missing MODULE_DESCRIPTION() macros

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/iio/adc/ingenic-adc.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/iio/adc/xilinx-ams.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/iio/buffer/kfifo_buf.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240607-md-drivers-iic-v1-1-9f9db6246083@quicinc.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>