sfc: Move RX data FIFO thresholds out of struct efx_nic_type

Since there are now separate blocks of code to set the thresholds for
each NIC type, it is no longer useful to include them in the NIC type
description.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>