net: xilinx: Init mdio_mcreg variable

net: xilinx: Init mdio_mcreg variable

Init variable which is not used for 10G/25G that's why compiler
doesn't see it to be initialized properly which is causing
compilation warning:
drivers/net/ethernet/xilinx/xilinx_axienet_main.c: In function
'axienet_dma_err_handler':
drivers/net/ethernet/xilinx/xilinx_axienet_main.c:2117:3: warning:
'mdio_mcreg' may be used uninitialized in this function
[-Wmaybe-uninitialized]
   axienet_iow(lp, XAE_MDIO_MC_OFFSET, mdio_mcreg);

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