staging: iio: tsl2x7x: add common function for clearing interrupts

There were three places where the same chunk of code was used to clear
interrupts. This patch creates a common function
tsl2x7x_clear_interrupts() to reduce duplicate code.

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