leds: Remove uneeded strlen calls

There's no need for the additional call to strlen(), we can directly
return the value returned by sprintf().  We now return a length value
that doesn't include the final '\0', but user space shouldn't bother
about it anyway.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>