iio: Use octal permissions and DEVICE_ATTR_{RO,RW}.

As reported by checkpatch.pl.  Where possible use DEVICE_ATTR_RO(),
DEVICE_ATTR_RW(), and __ATTR_RO().  Change function names to be
<var>_show() for read and <var>_store() for write.

Suggested-by: Joe Perches <joe@perches.com>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Joe Simmons-Talbott <joetalbott@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220601185414.251571-1-joetalbott@gmail.com


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