V4L/DVB (13232): cx25840 6.5MHz carrier detection fixes

cx25840:
Disable 6.5MHz carrier autodetection for PAL, always assume its DK.
Only try to autodetect 6.5MHz carrier for SECAM if user accepts both
system DK and L.

Signed-off-by: Aleksandr V. Piskunov <alexandr.v.piskunov@gmail.com>
Reviewed-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>