iio:light:tsl2563 use generic fw accessors

iio:light:tsl2563 use generic fw accessors

Replace of_property_read_u32() with device_property_read_u32(),
when reading the amstaos,cover-comp-gain.This opens up the
possibility of passing the properties during platform instantiation
of the device by a suitable populated struct property_entry.
Additionally, a minor change in logic is added to remove the
of_node present check.

Signed-off-by: Vaishnav M A <vaishnav@beagleboard.org>
Link: https://lore.kernel.org/r/20201018203552.GA816421@ubuntu
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>