leds: heartbeat: stop on shutdown

A halted kernel should not show a heartbeat.

[akpm@linux-foundation.org: checkpatch fixes]
Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Cc: Shuah Khan <shuahkhan@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>