sfc: Do not attempt to flush queues if DMA is disabled

[ Upstream commit 3dca9d2dc285faf1910d405b65df845cab061356 ]

efx_nic_fatal_interrupt() disables DMA before scheduling a reset.
After this, we need not and *cannot* flush queues.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>