input: touchscreen: synaptics_dsx: Generate the hardware ts on interrupt

We were generating the hardware timestamps when processing the workqueue,
which can happen "much" later than the actual irq. Increase the ts
reporting precision by generating the timestamp on the interrupt handler,
and get rid of the work queue entirely.

Change-Id: I171aad3d4a8f40eb7f8ab40abea0e9f1df2f1879