nl80211: Missing sysctl flags aren't fatal

nl80211: Missing sysctl flags aren't fatal

The relevant flags were only added in Linux 4.6, so we shouldn't
complain because they're missing. Also, they're always missing if a
device is being removed (e.g., 'iw dev wlan0 del', or if the device is
in the process of resetting itself). So kill those 2 birds with 1 stone:
if we can't find the file, just silently skip it.

Also, we probably should *actually* propagate the error if we had a
write failure.

Signed-off-by: Brian Norris <briannorris@chromium.org>