Replace the PPP workqueue with a dedicated thread

The PPP driver has little work to do but it is critical that it be
completed on time. With a dedicated thread we can use the realtime
scheduler to achieve the desired performance characteristics.

This is modeled after the behavior of the display thread in the
mdss_fb driver.

Change-Id: I599928d5f2c581718296be03b0ba14c6658fdeea
1 file changed