tsc2007: remove HR timer, make platform callbacks optional

This patch removes the HR timer, since it's bad to do synchronous I2C in the HR timer callback context. The new implementation makes use of the global workqueue. The work is scheduled every 5ms when polling rather than 5 us.

Secondly the platform callbacks are only called if supplied. Makes the driver to fallback on only pressure calculation to decide when the pen is up. This is not as accurate as having a function to call, but that's not possible on all hardware configurations.

--Richard