tools/hotplug/Linux: don't needlessly use non-standard features in vif-{bridge,route}

tools/hotplug/Linux: don't needlessly use non-standard features in vif-{bridge,route}

We're not after any "fall-through" behavior here. Replace the constructs
with ones understood by all conforming shells, including older bash
(problem observed with 3.1.51(1)).

Fixes: b51715f02bf9 ("tools/hotplug/Linux: remove code duplication in vif-bridge")
Fixes: 3683290fc0b0 ("tools/hotplug: only attempt to call 'ip route' if there is valid command")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Wei Liu <wl@xen.org>