xilinx_ethlite: fix eth_can_rx() for ping-pong

The eth_can_rx() function only checks the first buffers status ("ping"). The
controller should be able to receive into "pong" when ping-pong is enabled.
Checks the active buffer (either "ping" or "pong") when determining can_rx()
rather than just testing "ping".

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
