atl1c: remove private tx lock

AFAICS this is safe: the lock is only used in the .ndo_start_xmit
function and this driver does not set LLTX.

Gets rid of TX_LOCKED return value, followup patches will remove it.

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>