Use link speed from device capability instead of negotiated speeda

Use link speed from device capability instead of negotiated speeda

JIRA: SAMPLEVNF-151

link speed is used in gen and lat latency extrapolations.
Using a link_speed value lower than the actual link speed
might result in errors (e.g. negative latencies).
Negotiated link speed might be reported slowly (as reported through irq)
Hence it is better to use the device capability link speed.
In addition, this remove the check for link speed changes in fastpath.

Change-Id: I0f475fe5e139b046012de6cd0b710e4390735078
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>