tsnep: Replace TX spin_lock with __netif_tx_lock

TX spin_lock can be eliminated, because the normal TX path is already
protected with __netif_tx_lock and this lock can be used for access to
queue outside of normal TX path too.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>