e1000: conditionally raise irq at the end of MDI cycle

According to the spec:

"When set to 1b by software, it causes an Interrupt to be
asserted to indicate the end of an MDI cycle."

We need check the Interrupt Enable bit and raise irq only when it is
set.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
