ibmvnic: fix firmware version when no firmware level has been provided by the VIOS server

Older versions of VIOS servers do not send the firmware level in the VPD
buffer for the ibmvnic driver. Thus, not only the current message is mis-
leading but the firmware version in the ethtool will be NULL. Therefore,
this patch fixes the firmware string and its warning.

Fixes: 4e6759be

 ("ibmvnic: Feature implementation of VPD for the ibmvnic driver")
Signed-off-by: Desnes A. Nunes do Rosario <desnesn@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>