vmxnet3: fix publicity of NETIF_F_HIGHDMA

NETIF_F_HIGHDMA is being disabled even when dma64 is true. This patch fixes it.

CC: Michal Miroslaw <mirq-linux@rere.qmqm.pl>
Signed-off-by: Shreyas N Bhatewara <sbhatewara@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>