Changes

netdev: Explicit cast time() to uint32_t seq_nr

Make coverity happy and also inform readers of the code that we are fine
dropping the most significat part of the return value of time() when
using it to set up a nl seq_nr.

Related: Coverity CID#426078
Change-Id: Ia4c1c9b2dcc751b6cbd01a957627f882bc63ba87