atomisp/css: optimize sh_css_hrt_irq_get_id()

atomisp/css: optimize sh_css_hrt_irq_get_id()

BZ: 23514

There is no need to loop to find the first set bit. We can easily use the ffs()
function to do the job. And this way is faster.

This patch implements faster and simplier codepiece.

Change-Id: Id9d26a0cbf3c9a965e0c82fbd47d47734730df29
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-on: http://android.intel.com:8080/34932
Reviewed-by: Shevchenko, Andriy <andriy.shevchenko@intel.com>
Reviewed-by: Kruger, Jozef <jozef.kruger@intel.com>
Reviewed-by: Wang, Wen W <wen.w.wang@intel.com>
Reviewed-by: Koski, Anttu <anttu.koski@intel.com>
Tested-by: Koski, Anttu <anttu.koski@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>