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.
Fixes: aa3b6a08c5 ("kernel: Replace ledtrig-netdev with upstream backport")
Reported-by: Hannu Nyman <hannu.nyman@iki.fi>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>