sfc: reduce severity of PIO buffer alloc failures

PIO buffer allocation can fail for two valid reasons:
 - we've run out of them (results in -ENOSPC)
 - the NIC configuration doesn't support them (results in -EPERM)
Since both these failures are expected netif_err is excessive.

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