enic: Fix sparse warning in vnic_devcmd_init().

>> drivers/net/ethernet/cisco/enic/vnic_dev.c:1095:13: sparse: incorrect type in assignment (different address spaces)
   drivers/net/ethernet/cisco/enic/vnic_dev.c:1095:13:    expected void *res
   drivers/net/ethernet/cisco/enic/vnic_dev.c:1095:13:    got void [noderef] <asn:2>*

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>