sfc: ef100 representor RX NAPI poll

This patch adds the 'bottom half' napi->poll routine for representor RX.
See the next patch (with the top half) for an explanation of the 'fake
 interrupt' scheme used to drive this NAPI context.

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