ipam: Fix error strong when router IP is not available
ipam: Fix error strong when router IP is not available

The error is returned for all address families, the existing gives a wrong
indication that the IPv4 router IP cannot be allocated.

Signed-off-by: Thomas Graf <5f50a84c1fa3bcff146405017f36aec1a10a9e38@cilium.io>

michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,cilium-team/cilium,tklauser/cilium,michi-covalent/cilium,cilium-team/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium