net: axienet: Fix register defines comment description

In axiethernet header fix register defines comment description to be
inline with IP documentation. It updates MAC configuration register,
MDIO configuration register and frame filter control description.

Fixes: 8a3b7a25 ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>