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

commit 9aa5b0181bdf335f0b731d8502e128a862884bcd upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>