platform/x86: intel_punit_ipc: Avoid error message when retrieving IRQ

Since the commit

  7723f4c5 ("driver core: platform: Add an error message to platform_get_irq*()")

the platform_get_irq() started issuing an error message which is not
what we want here.

Switch to platform_get_irq_optional() to have only warning message
provided by the driver.

Fixes: 7723f4c5 ("driver core: platform: Add an error message to platform_get_irq*()")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>