ibmvnic: Fix incorrectly defined ibmvnic_request_map_rsp structure

This reserved area should be eight bytes in length instead of four.
As a result, the return codes in the REQUEST_MAP_RSP descriptors
were not being properly handled.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>