drivers/char/i8k.c: add Dell XPLS L421X

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=60772



Signed-off-by: Alan Cox <alan@linux.intel.com>
Reported-by: Leho Kraav <leho@kraav.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>