platform/x86: ISST: drop a duplicated word in isst_if.h

Drop the repeated word "for" in a comment.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: platform-driver-x86@vger.kernel.org
Cc: Darren Hart <dvhart@infradead.org>
Cc: Andy Shevchenko <andy@infradead.org>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>