sfc: Add explicit RX queue flag to channel

The PTP channel will have its own RX queue even though it's not
a regular traffic channel.

Original work by Ben Hutchings <bhutchings@solarflare.com>

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