tun: Eliminate sparse signedness warning

register_pernet_gen_device() expects 'int*', found via sparse.

 CHECK   drivers/net/tun.c
 drivers/net/tun.c:1245:36: warning: incorrect type in argument 1 (different signedness)
 drivers/net/tun.c:1245:36:    expected int *id
 drivers/net/tun.c:1245:36:    got unsigned int static [toplevel] *<noident>

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>