[media] staging/tm6000: Fix a warning message

I added a code to the driver to force it to produce a warning. This
were intended to remind me about a very bad hack. I never found a way
to workaround. So, instead of those warnings:

drivers/staging/tm6000/tm6000-core.c: In function ‘tm6000_init_analog_mode’:
drivers/staging/tm6000/tm6000-core.c:328: warning: ISO C90 forbids mixed declarations and code

Let's document the issue and hope if someone with the support of the vendor
might fix it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>