net: axienet: Fix compilation warnings

Warning log:
xilinx_axienet_main.c: In function 'axienet_start_xmit_done':
xilinx_axienet_main.c:617:16: warning: operation on 'lp->tx_bd_ci' may be undefined [-Wsequence-point]
xilinx_axienet_main.c: In function 'axienet_start_xmit':
xilinx_axienet_main.c:703:18: warning: operation on 'lp->tx_bd_tail' may be undefined [-Wsequence-point]
xilinx_axienet_main.c:719:17: warning: operation on 'lp->tx_bd_tail' may be undefined [-Wsequence-point]
xilinx_axienet_main.c: In function 'axienet_recv':
xilinx_axienet_main.c:792:16: warning: operation on 'lp->rx_bd_ci' may be undefined [-Wsequence-point]
xilinx_axienet_main.c: In function 'axienet_of_probe':
xilinx_axienet_main.c:1501:21: warning: unused variable 'rc' [-Wunused-variable]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>