8139cp: set intr mask after its handler is registered

We set intr mask before its handler is registered, this does not work well when
8139cp is sharing irq line with other devices. As the irq could be enabled by
the device before 8139cp's hander is registered which may lead unhandled
irq. Fix this by introducing an helper cp_irq_enable() and call it after
request_irq().

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>