vmxnet: trivial annotation of protocol constant

Noticed by sparse:
drivers/net/vmxnet3/vmxnet3_drv.c:876:38: warning: cast from restricted __be16
drivers/net/vmxnet3/vmxnet3_drv.c:876:38: warning: cast from restricted __be16
drivers/net/vmxnet3/vmxnet3_drv.c:876:24: warning: restricted __be16 degrades to integer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>