sfc: Tighten the check for RX merged completion events

The addition of RX event merging support means we don't reliably
detect dropped RX events now.  Currently we will only detect them if
the previous event for the RX queue had the CONT bit set.

Only accept RX completion events as merged if the
GET_CAPABILITIES_OUT_RX_BATCHING bit is set in datapath_caps (which it
won't be for the low-latency datapath) and the CONT bit is not set on
the event.

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