iio: adc: ad799x: change 'unsigned' to 'unsigned int' declaration

Prefer 'unsigned int' instead of bare use of 'unsigned' declarations to
to improve code readbility. This ceases one of the warning messages
pointed by checkpatch.

Co-developed-by: Bruna Lopes <brunaafl@usp.br>
Signed-off-by: Bruna Lopes <brunaafl@usp.br>
Signed-off-by: Gustavo Rodrigues <ogustavo@usp.br>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240428194326.2836387-2-ogustavo@usp.br


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