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 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc drivers/net/ethernet/sfc/efx.c
Simple merge

diff --cc drivers/net/ethernet/sfc/efx.h
Simple merge

diff --cc drivers/net/ethernet/sfc/selftest.c
Simple merge