Revert "net: ibm/emac: allocate dummy net_device dynamically"

This reverts commit b40eeedbbc0833716b5ccae64cf914f2604a3e5e, which was
upstream commit 2eb5e25d ("net: ibm/emac: allocate dummy net_device
dynamically").

alloc_netdev_dummy(( does not exist in 6.1, so all this backport did was
break the build.

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