iio: adc: ad799x: Prefer to use octal permission

Octal permissions are preferred over the symbolics ones
for readbility. This ceases warning message 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/20240428194326.2836387-4-ogustavo@usp.br


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