ipvlan: mode is u16

The mode argument was erronusly defined as u32 but it has always
been u16. Also use ipvlan_set_mode() helper to set the mode instead
of assigning directly. This should avoid future erronus assignments /
updates.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>