main: Make read timeout longer

We're targetting to have the device change orientation after being
in the same position for 800 msecs by default, to avoid mid-flight
changes.

So we don't need to check more often than every 800 msecs. Shave
about 100 msecs for the processing of the information.
main: Scan size never changes, so calculate it once

As the channels never change, the scan size never changes either,
so calculate it at the beginning of the program, and reuse that
value.
main: Don't create the device path in the loop

There's no need to do this in each and every loop, the device
path doesn't change from one loop to another.
main: Comment unused debug function

string_to_orientation() is unused for now.
main: Setup ring buffer and trigger on startup

This shouldn't be something we need to do for every loop, so
only do that on startup, and tear down on exit.
main: Replace printf warning by g_warning
main: Remove dead debug code