drivers: net: remove NETDEV_TX_LOCKED

replace the trylock by a full spin_lock and remove TX_LOCKED return value.
Followup patch will remove TX_LOCKED from the kernel.

Cc: Jon Mason <jdmason@kudzu.us>
Cc: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>