net/sfc/base: support the Rx event mode w/o continue

net/sfc/base: support the Rx event mode w/o continue

The recently added NO_CONT_EV mode is recommended for when looking for
maximum throughput on 100G links as it allows the firmware to operate
more efficiently. The biggest benefit is when using scatter and jumbo
frames, but it is also necessary to achieve line rate in other cases.

Support for NO_CONT_EV when scatter is disabled is simple - the main
datapath change is to always read the packet length from the prefix,
not the event. This requires storing a flag with the event queue so
the event handler knows NO_CONT_EV mode is in use.

Supporting NO_CONT_EV with scatter would require an API change.

(Now the ee_flags field in efx_evq_t is used on the datapath, move it
next to other fields which are also read on the datapath to try to avoid
reading an additional cache line.)

Signed-off-by: Mark Spender <mspender@solarflare.com>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>