staging: iio: light: Replace symbolic permissions as per coding style

Fixed the following checkpatch.pl warnings:
octal permissions are more preferable than symbolic permissions

Replaced DEVICE_ATTR family macros with DEVICE_ATTR_RW family
as suggested by Greg K-H. Changed attributes and function
names where ever required to satisfy internal macro definitions
like __ATTR__RW().

Signed-off-by: Surender Polsani <surenderpolsani@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>