net/e1000: always enable receive and transmit

net/e1000: always enable receive and transmit

The transmit and receive controller state machines are only enabled after
receiving an interrupt and the link status is now valid.  If an adapter
is being used in conjunction with NC-SI, network controller sideband
interface, the adapter may never get a link state change interrupt since
the adapter's PHY is always link up and never changes state.

To fix this, always enable and disable the transmit and receive with
.dev_start and .dev_stop.  This is a better match for what is typically
done with the other PMD's.  Since we may never get an interrupt to check
the link state, we also poll once at the end of .dev_start to get the
current link status.

Signed-off-by: Chas Williams <chas3@att.com>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>