tools/power turbostat: read from pipes too

Commit '47936f94 tools/power turbostat: fix printing on input' make
a valid fix, but it completely disabled piped stdin support, which is
a valuable use-case. Indeed, if stdin is a pipe, turbostat won't read
anything from it, so it becomes impossible to get turbostat output at
user-defined moments, instead of the regular intervals.

There is no reason why this should works for terminals, but not for
pipes. This patch improves the situation. Instead of ignoring pipes, we
read data from them but gracefully handle the EOF case.

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