V4L/DVB (6173): cx23885: Minor cleanup and important NMI comment placed in code

I wanted to document the NMI assert issue inside the code, even though
it's already documented in the patch history. If/when the next cx23887
revision appears, is may need to be enabled on that also.

Signed-off-by: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>