[TG3]: Remove unnecessary tx_lock

Remove tx_lock where it is unnecessary. tg3 runs lockless and so it
requires interrupts to be disabled and sync'ed, netif_queue and NAPI
poll to be stopped before the device can be reconfigured. After
stopping everything, it is no longer necessary to get the tx_lock.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>