sfc: ensure recovery after allocation failures

After failing to allocate a receive buffer the driver may fail to ever
request additional allocations. EF10 NICs require new receive buffers to
be pushed in batches of eight or more. The test for whether a slow fill
should be scheduled failed to take account of this. There is little
downside to *always* requesting a slow fill if we failed to allocate a
buffer, so the condition has been removed completely. The timer that
triggers the request for a refill has also been shortened.

Signed-off-by: Robert Stonehouse <rstonehouse@solarflare.com>
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>