V4L/DVB: IR/imon: protect ictx's kc and last_keycode w/spinlock

Lest we get our keycodes wrong... Thus far, in practice, I've not found
it to actually matter, but its one of the issues raised in
https://bugzilla.kernel.org/show_bug.cgi?id=16351

 that wasn't addressed
by converting to using native IR keydown/up functions.

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>