staging: iio: tsl2x7x: correctly return errors in tsl2x7x_get_prox()

Not all errors that occurred in tsl2x7x_get_prox() were correctly
reported in the return value. This patch changes the error handling
so that errors are now returned properly.

Note that the ret variable is from the call to tsl2x7x_read_status(),
and it already has the correct error check. The -EINVAL error code is
for an unexpected value in the register.

This patch also corrects an unnecessary word wrap in the call to
le16_to_cpup() while changes are being made to this code.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>