tools/power/x86/intel-speed-select: Helpful warning for missing kernel interface

When the device file "/dev/isst_interface" is not present, instead of
failing on access, check at the start and print a helpful warning.

Here CLX platform is an exception, which doesn't depend on the device
file. So continue for CLX platform.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>