efx_device_detach_sync() locks all TX queues before marking the device
detached and thus disabling further TX scheduling.  But it can still
be interrupted by TX completions which then result in TX scheduling in
soft interrupt context.  This will deadlock when it tries to acquire
a TX queue lock that efx_device_detach_sync() already acquired.

To avoid deadlock, we must use netif_tx_{,un}lock_bh().
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>