ibmvnic: fix empty firmware version and errors cleanup

This patch makes sure that the firmware version is never NULL. Moreover,
it also performs some cleanup on the error messages.

Fixes: a107311d ("ibmvnic: fix firmware version when no firmware level
has been provided by the VIOS server")
Signed-off-by: Desnes A. Nunes do Rosario <desnesn@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>