similar to atl1c: lock is only used in ndo_start_xmit, but we also
advertised LLTX, so remove that as well and let core stack handle
tx locking.

Allows to remove the TX_LOCKED return value from the driver.

Cc: Jay Cliburn <jcliburn@gmail.com>
Cc: Chris Snook <chris.snook@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>