staging: iio: ad9832: kernel-doc fixes

Fixes a W=1 warning.
-Added ``:`` to lock parameter in 'ad9832_state' description.
-It's a reference comment so removed /**

Signed-off-by: Mugilraj Dhavachelvan <dmugil2000@gmail.com>
Link: https://lore.kernel.org/r/20210315133711.26860-1-dmugil2000@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>