platform/x86: intel_mid_powerbtn: Convert to use new SCU IPC API

This converts the power button driver to use the new SCU IPC API where
the SCU IPC instance is passed to the functions.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>