hv_netvsc: Add initialization of tx_table in netvsc_device_add()

tx_table is part of the private data of kernel net_device. It is only
zero-ed out when allocating net_device.

We may recreate netvsc_device w/o recreating net_device, so the private
netdev data, including tx_table, are not zeroed. It may contain channel
numbers for the older netvsc_device.

This patch adds initialization of tx_table each time we recreate
netvsc_device.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>