tests: interrupt_util: Update RX irq trigger method

The QEMU RX doesn't allow to write value to IR flag,
There is also mentioned in RX HWM that the IR should
not be write 1 to. So we change the method to trigger
interrupt here is to call directly to SW irq table

Signed-off-by: Duy Nguyen <duy.nguyen.xa@renesas.com>