netvsc: Set maximum GSO size in the right place

Commit a50af86d "netvsc: reduce maximum GSO size" was wrongly
backported to 4.4-stable.  The maximum size needs to be set before the
net device is registered, in netvsc_probe().

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>