In the upstream netdev led trigger the one mode file was replaced by 3
files named rx, tx and link. Fix the netdev trigger configuration code
to use the modified API.
This fix is based on 201058b35c ("base-files: Fix netdev led trigger")
Fixes: aa3b6a08c5 ("kernel: Replace ledtrig-netdev with upstream backport")
Signed-off-by: Martin Schiller <ms@dev.tdt.de>