tools/power turbostat: fix printing on input

The recent patch that implements table printing on a keypress introduced a
regression - turbostat prints the table almost continuously if it is run from a
daemon program.

The problem is also easy to reproduce like this:

echo | turbostat

The reason is that we cannot assume that stdin is always a TTY. It can be many
things.

This patch adds fixes the problem by limiting the new keypress functionality to
TTYs only. If stdin is not a TTY, we just sleep for the full interval time.

While on it, clean-up 'do_sleep()' to return no value, as callers do not expect
that anyway.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>