procd: reduce the number of unnecessary calls to /sbin/hotplug-call for tty devices

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38705 3c298f89-4303-0410-b956-a3cf2f4a3e73