ibmveth: enable interrupts after napi_complete()

The interrupt is enabled before napi_complete(). A network timeout
occurs if the interrupt handler is called before napi_complete().

Fix the bug by enabling the interrupt after napi_complete().

Signed-off-by: Yongbae Park <yongbae2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>