V4L/DVB (12338): cx18: Read buffer overflow

This mistakenly tested against sizeof(freqs) instead of the array size. Due to
the mask the only illegal value possible was 3.

Reported-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>