staging:iio:tsl2x7x: Use iio_str_to_fixedpoint instead of open-coding it

The tsl2x7x driver has a copy'n'pasted version of the iio_str_to_fixedpoint()
function from the IIO core. Replace this custom copy and use
iio_str_to_fixedpoint instead.

The patch also introduces a slight functional change in that it makes sure that
in case of a parsing error the error is reported back to userspace instead of
silently ignoring it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jon Brenner <jon.brenner@ams.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>