ath5k: ensure led name is null terminated

Add the missing null termination after strncpy().

This isn't actually a buffer overflow in this case since we use
snprintf() appropriately to fill the buffer passed by the caller,
but in the interest of not turning this into a bug down the road,
go ahead and force termination here.

Found by Coverity.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>