vmxnet3: prevent div-by-zero panic when ring resizing uninitialized dev

Linux is free to call ethtool ops as soon as a netdev exists when probe
finishes. However, we only allocate vmxnet3 tx/rx queues and initialize the
rx_buf_per_pkt field in struct vmxnet3_adapter when the interface is
opened (UP).

Signed-off-by: Bhavesh Davda <bhavesh@vmware.com>
Signed-off-by: Shreyas N Bhatewara <sbhatewara@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>