b43legacy/leds: Ensure NUL-termination of LED name string

strncpy might not NUL-terminate the string, if the name equals the buffer size.
Use strlcpy instead.

Signed-off-by: Michael Buesch <m@bues.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>