p910nd: update default config to point at /dev/usb/lp0 instead of /dev/lp0 (#11968)

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

git-svn-id: svn://svn.openwrt.org/openwrt/packages@36536 3c298f89-4303-0410-b956-a3cf2f4a3e73