hyperv: NULL dereference on error

We try to call free_netvsc_device(net_device) when "net_device" is NULL.
It leads to an Oops.

Fixes: f90251c8

 ('hyperv: Increase the buffer length for netvsc_channel_cb()')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>