platform/x86: intel_scu_ipc: Sleeping is fine when polling

There is no reason why the driver would need to block other threads from
running the CPU while it is waiting for the SCU IPC to complete its
work. For this reason switch the driver to use usleep_range() instead
with a bit more relaxed polling loop.

Also add constant for the timeout and use the same value for both
polling and interrupt modes.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>