sfc: Use netif_device_{detach,attach}() around reset and self-test

We need to keep the TX queues stopped throughout a reset, without
triggering the TX watchdog and regardless of the link state.  The
proper way to do this is to use netif_device_{detach,attach}() just as
we do around suspend/resume, rather than the current bodge of faking
link-down.

Since we also need to do this during an offline self-test and we
perform a reset during that, add these function calls outside of
efx_reset_down() and efx_reset_up().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>