V4L/DVB (12166): cx23885: add FIXME comment above set_frontend override

add FIXME comment to indicate that the set_frontend override is a
temporary hack.  This will be done a better way in the next kernel.

Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>