ibmvnic: fix kernel build warning in strncpy

drivers/net/ethernet/ibm/ibmvnic.c: In function ‘handle_vpd_rsp’:
drivers/net/ethernet/ibm/ibmvnic.c:4393:3: warning: ‘strncpy’ output truncated before terminating nul copying 3 bytes from a string of the same length [-Wstringop-truncation]
 4393 |   strncpy((char *)adapter->fw_version, "N/A", 3 * sizeof(char));
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Lijun Pan <lijunp213@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>