led: fix up the ENOSYS condition to pass coverity

The current ENOSYS check code can proceed for a driver that
hypothetically implements the on() but not the off() function of
vice versa, which would result in a null pointer dereference.

This would be a weird use case but Coverity catches the situation, no
harm in changing the ENOSYS check to fail if the on/off is half
implemented, so let's change it to do that.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>