procd: Fix USB cellular modems hotplug events

Without this patch, /etc/hotplug.d/tty/30-3g won't be called when you insert or remove a USB-serial cellular modem.

(see Ticket #14117)

Signed-off-by: Matthew Reeve <mreeve@tenxnetworks.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41039 3c298f89-4303-0410-b956-a3cf2f4a3e73