net: fman: Unregister ethernet device on removal

When the mac device gets removed, it leaves behind the ethernet device.
This will result in a segfault next time the ethernet device accesses
mac_dev. Remove the ethernet device when we get removed to prevent
this. This is not completely reversible, since some resources aren't
cleaned up properly, but that can be addressed later.

Fixes: 39339616

 ("fsl/fman: Add FMan MAC driver")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Link: https://lore.kernel.org/r/20221103182831.2248833-1-sean.anderson@seco.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>