sfc: implement ethtool get/set RX ring size for EF100 reps

It's not truly a ring, but the maximum length of the list of queued RX
 SKBs is analogous to an RX ring size, so use that API to configure it.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>