xen-netback: make ops structs const

All tables of function pointers should be const to make hacks
more difficult. Compile tested only.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>