media: ddbridge/mci: protect against out-of-bounds array access in stop()

In stop(), an (unlikely) out-of-bounds write error can occur when setting
the demod_in_use element indexed by state->demod to zero, as state->demod
isn't checked for being in the range of the array size of demod_in_use, and
state->demod maybe carrying the magic 0xff (demod unused) value. Prevent
this by checking state->demod not exceeding the array size before setting
the element value. To make the code a bit easier to read, replace the magic
value and the number of array elements with defines, and use them at a few
more places.

Detected by CoverityScan, CID#1468550 ("Out-of-bounds write")

Thanks to Colin for reporting the problem and providing an initial patch.

Fixes: daeeb131 ("media: ddbridge: initial support for MCI-based MaxSX8 cards")

Reported-by: Colin Ian King <colin.king@canonical.com>
Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>