tools/power/x86/intel-speed-select: Remove warning for unused result

Fix warning for:
isst-config.c: In function ‘set_cpu_online_offline’:
isst-config.c:221:3: warning: ignoring return value of ‘write’,
declared with attribute warn_unused_result [-Wunused-result]
   write(fd, "1\n", 2);

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