V4L/DVB (13902): cx18: Update MPEG Index stream buffers module option processing

Update the module options related to INDEX stream buffer allocation.  A single
CX2341[5678] index entry is only 24 bytes.  Large buffers for the IDX stream
will prevent the CX23418 from transferring index data over at all.  Buffers
of around 1.5 kB or 64 index entries seem to be just fine.  We'll default to
63 buffers/MDLs as that is the firmware limit per stream and IDX stream buffers
are not high rate.  There is no reason on earth to allocate the previous 1 MB
default of buffer space for the IDX stream.

This is in anticipation of implementing the G_ENC_INDEX ioctl() in the cx18
driver.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>