net/ncsi: Fix length of GVI response packet

The length of GVI (GetVersionInfo) response packet should be 40 instead
of 36. This issue was found from /sys/kernel/debug/ncsi/eth0/stats.

 # ethtool --ncsi eth0 swstats
     :
 RESPONSE     OK       TIMEOUT  ERROR
 =======================================
 GVI          0        0        2

With this applied, no error reported on GVI response packets:

 # ethtool --ncsi eth0 swstats
     :
 RESPONSE     OK       TIMEOUT  ERROR
 =======================================
 GVI          2        0        0

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>