i40e: add helper conversion function for link_speed

We introduced the virtchnl interface in order to have an interface for
talking to a virtual device driver which was host-driver agnostic. This
interface has its own definitions, including one for link speed.

The host driver has to talk to the virtchnl interface using these new
definitions in order to remain compatible. Today, the i40e link_speed
enumerations are value-exact matches for the virtchnl interface, so it
was originally decided to simply use a typecast.

However, this is unsafe, and makes it easier for future drivers to
continue this unsafe practice. There is nothing guaranteeing these
values are exact, and the type-cast would hide any compiler warning
which indicates the problem.

Rather than rely on this type cast, introduce a helper function which
can convert the AdminQ link speed definition into a virtchnl
definition. This can then be used by host driver implementations in
order to safely convert to the interface recognized by the virtual
functions.

If the link speed is not able to be represented by the virtchnl
definitions we'll report UNKNOWN which is the safest result.

This will ensure that should the driver specific link_speeds actual bit
definitions change, we do not report them incorrectly according to the
VF.

Additionally, this provides a better pattern for future drivers to copy,
as it is more likely a future device may not use the exact same bit-wise
definition as the current virtchnl interface.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>