[VLAN]: Avoid expensive divides

We can avoid divides (as seen with CONFIG_CC_OPTIMIZE_FOR_SIZE=y on
x86) changing vlan_group_get_device()/vlan_group_set_device() id
parameter from signed to unsigned.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>