net: axienet: Fix register defines comment description

[ Upstream commit 9ff2f816e2aa65ca9a1cdf0954842f8173c0f48d ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>