sfc: Add PM and RXDP drop counters to ethtool stats

Recognise the new Packet Memory and RX Data Path counters.

The following counters are added:
rx_pm_{trunc,discard}_bb_overflow - burst buffer overflowed.  This should not
 occur if BB correctly configured.
rx_pm_{trunc,discard}_vfifo_full - not enough space in packet memory.  May
 indicate RX performance problems.
rx_pm_{trunc,discard}_qbb - dropped by 802.1Qbb early discard mechanism.
 Since Qbb is not supported at present, this should not occur.
rx_pm_discard_mapping - 802.1p priority configured to be dropped.  This should
 not occur in normal operation.
rx_dp_q_disabled_packets - packet was to be delivered to a queue but queue is
 disabled.  May indicate misconfiguration by the driver.
rx_dp_di_dropped_packets - parser-dispatcher indicated that a packet should be
 dropped.
rx_dp_streaming_packets - packet was sent to the RXDP streaming bus, ie. a
 filter directed the packet to the MCPU.
rx_dp_emerg_{fetch,wait} - RX datapath had to wait for descriptors to be
 loaded.  Indicates performance problems but not drops.

These are only provided if the MC firmware has the
PM_AND_RXDP_COUNTERS capability.  Otherwise, mask them out.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>