ibmvnic: Warn unknown speed message only when carrier is present

With commit 0655f994 ("net/ibmvnic: Update carrier state after link
state change") we are now able to detect when the carrier is properly
present in the device, so only report an unexpected unknown speed when it
is properly detected. Unknown speed is expected to be seen by the device
in case the backing device has no link detected.

Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Tested-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Signed-off-by: Murilo Fossa Vicentini <muvic@linux.ibm.com>
Reviewed-by: Thomas Falcon <tlfalcon@linux.ibm.com>