net: emaclite: Use indirect access in emaclite_recv

When IP is configured with pong buffers, IP is receiving packets to ping
and then to pong buffer and than ping again.
The original logic in the driver remains there that when ping buffer is
free, pong buffer is checked too and return if both are free.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>