net/virtio: invoke method directly for setting IRQ config

We need to define a prototype for such wrapper, which makes thing
too complicated. Remove wrapper and call set_config_irq directly.

Suggested-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Tested-by: Lei Yao <lei.a.yao@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>