sfc: Skip RX end-of-batch work on channels without an RX queue

The code in efx_process_channel() to update the RX queue after each
batch of RX completions works out as a no-op on a TX-only channel
where the RX queue structure is set to all-zeroes, but
(1) efx_channel_get_rx_queue() will BUG() if DEBUG is defined, and
(2) it's a waste of time.

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