Input: wm831x-ts - move BTN_TOUCH reporting to data transfer

Don't report BTN_TOUCH until we've got data as some less robust applications
can be confused by getting a touch event by itself and it doesn't seem
unreasonable for them to expect coordinates along with a touch.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>