net: macvlan: Demote nonconformant function header

net: macvlan: Demote nonconformant function header

Fixes the following W=1 kernel build warning(s):

 drivers/net/macvlan.c:1350: warning: Function parameter or member 'vlan' not described in 'macvlan_changelink_sources'
 drivers/net/macvlan.c:1350: warning: Function parameter or member 'mode' not described in 'macvlan_changelink_sources'
 drivers/net/macvlan.c:1350: warning: Function parameter or member 'data' not described in 'macvlan_changelink_sources'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201102114512.1062724-27-lee.jones@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>