Otherwise, if we fail to allocate new PIO buffers, our TXQs will try to
use the old ones, which aren't there any more.

Fixes: 183233be "sfc: Allocate and link PIO buffers; map them with write-combining"
Signed-off-by: NEdward Cree <ecree@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>