media: Don't let tvp5150_get_vbi() go out of vbi_ram_default array

As pointed by Dan, possible values for bits[3:0] of te Line Mode Registers
can range from 0x0 to 0xf, but the check logic allow values ranging
from 0x0 to 0xe.

As static arrays are initialized with zero, using a value without
an explicit initializer at the array won't cause any harm.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>