iio: fix SPHINX kernel-docs build warning

When building kernel documentation sphinx emits the following warnings

No description found for parameter 'iio_dev'
Excess function parameter 'indio_dev' description in 'iio_device_register'

These warnings are caused by a macro with a different argument identifier to the
one listed in the kernel-docs.

Change macro argument to be the same as the docs.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>