net/fsl: xgmac_mdio: Fix incorrect iounmap when removing module

As reported by sparse: In the remove path, the driver would attempt to
unmap its own priv pointer - instead of the io memory that it mapped
in probe.

Fixes: 9f35a734 ("net/fsl: introduce Freescale 10G MDIO driver")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>