evdevtouch: Don't try to keep reading on EAGAIN

This stops touching the screen (and not moving) consuming 80-90% CPU.
The mtdev and non-mtdev codepaths have been separated for additional clarity.

Change-Id: I0559a6bd80dab961fdb4a83ad50860a9aec6445c
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>