netdevice: add helper to update trans_start

trans_start exists twice:
- as member of net_device (legacy)
- as member of netdev_queue

In order to get rid of the legacy case, add a helper for the
dev->trans_update (this patch), then convert spots that do

dev->trans_start = jiffies

to use this helper (next patch).

This would then allow us to change the helper so that it updates the
trans_stamp of netdev queue 0 instead.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>