drivers: replace dev->trans_start accesses with dev_trans_start

a trans_start struct member exists twice:
- in struct net_device (legacy)
- in struct netdev_queue

Instead of open-coding dev->trans_start usage to obtain the current
trans_start value, use dev_trans_start() instead.

This is not exactly the same, as dev_trans_start also considers
the trans_start values of the netdev queues owned by the device
and provides the most recent one.

For legacy devices this doesn't matter as dev_trans_start can cope
with netdev trans_start values of 0 (they are ignored).

This is a prerequisite to eventual removal of dev->trans_start.

Cc: linux-rdma@vger.kernel.org
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>