Input: ads7846 - fix race that causes missing releases

Input: ads7846 - fix race that causes missing releases

If touchscreen is released while busy reading HWMON device, the release
can be missed. The IRQ thread is not started because no touch is active
and BTN_TOUCH release event is never sent.

Fixes: 11518efcfe6352ea ("Input: ads7846 - avoid pen up/down when reading hwmon")
Co-developed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20201027105416.18773-1-o.rempel@pengutronix.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>