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>

Author: Gustavo Rodrigues

drivers/iio/adc/ad799x.c

@@ -524,7 +524,7 @@ static irqreturn_t ad799x_event_handler(int irq, void *private)
 	return IRQ_HANDLED;
 }
 
-static IIO_DEV_ATTR_SAMP_FREQ(S_IWUSR | S_IRUGO,
+static IIO_DEV_ATTR_SAMP_FREQ(0644,
 			      ad799x_read_frequency,
 			      ad799x_write_frequency);
 static IIO_CONST_ATTR_SAMP_FREQ_AVAIL("15625 7812 3906 1953 976 488 244 0");