sfc: Fold efx_flush_all() into efx_stop_port() and update comments

efx_flush_all() is a really misleading name - it has nothing to do
with e.g. flushing DMA queues.  Since it's called immediately after
efx_stop_port() and is highly dependent on what that does, combine
the two functions.

Update comments to explain what this is doing a little better.
Also update an related and erroneous comment in efx_start_port().

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