sfc: get PIO buffer size from the NIC

The 8000 series SFC NICs have 4K PIO buffers, rather than the 2K of
 the 7000 series.  Rather than having a hard-coded PIO buffer size
 (ER_DZ_TX_PIOBUF_SIZE), read it from the GET_CAPABILITIES_V2 MCDI
 response.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>