iio: adc: ad7173: Fix uninitialized symbol is_current_chan

Move the definition down and make it a boolean that is initialized
to false.

Fixes: 13d12e3a ("iio: adc: ad7173: Add support for AD411x devices")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202406232046.DKfBJq6o-lkp@intel.com/


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