Input: bu21013_ts - use interrupt from I2C client

Instead of trying to map INT GPIO to interrupt, let's use one supplied by
I2C client. If there is none - bail. This will also allow us to treat INT
GPIO as optional, as per the binding.

Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>