ethdev: allow unknown link speed

ethdev: allow unknown link speed

When querying the link information, the link status is
a mandatory major information.
Other boolean values are supposed to be accurate:
	- duplex mode (half/full)
	- negotiation (auto/fixed)

This API update is making explicit that the link speed information
is optional.
The value ETH_SPEED_NUM_NONE (0) was already part of the API.
The value ETH_SPEED_NUM_UNKNOWN (infinite) is added to cover
two different cases:
	- speed is not known by the driver
	- device is virtual

Suggested-by: Morten Brørup <mb@smartsharesystems.com>
Suggested-by: Benoit Ganne <bganne@cisco.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>