adv7842: comment out a table useful for debug

Gcc 6.1 warns about an unused table:

drivers/media/i2c/adv7842.c:2400:27: warning: 'prim_mode_txt' defined but not used [-Wunused-const-variable=]
 static const char * const prim_mode_txt[] = {
                           ^~~~~~~~~~~~~

That seems to be useful for debug, and likely were used before.
While we could simply remove, let's comment it out, for now.

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