platform/x86: intel_mid_powerbtn: Set IRQ_ONESHOT

The commit 1c6c6952 ("genirq: Reject bogus threaded irq requests")
starts refusing misconfigured interrupt handlers. This makes
intel_mid_powerbtn not working anymore.

Add a mandatory flag to a threaded IRQ request in the driver.

Fixes: 1c6c6952 ("genirq: Reject bogus threaded irq requests")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>