ibmvnic: Only retrieve error info if present

When handling a fatal error in the driver, there can be additional
error information provided by the vios. This information is not
always present, so only retrieve the additional error information
when present.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>