sfc: lock TX queues when calling netif_device_detach()

[ Upstream commit c2f3b8e3a44b6fe9e36704e30157ebe1a88c08b1 ]

The assertion of netif_device_present() at the top of
efx_hard_start_xmit() may fail if we don't do this.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
[bwh: Backported to 3.4: adjust context]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>