staging: iio: tsl2x7x: simplify tsl2x7x_write_interrupt_config return

tsl2x7x_write_interrupt_config() has an unnecessary return value check
at the end of the function. This patch changes the function to just
return the value from the call to tsl2x7x_invoke_change().

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