r820t: comment out two ancillary tables

As Gcc6.1 warned, those tables are currently unused:
	drivers/media/tuners/r820t.c:349:18: warning: 'r820t_mixer_gain_steps' defined but not used [-Wunused-const-variable=]
	 static const int r820t_mixer_gain_steps[]  = {
	                  ^~~~~~~~~~~~~~~~~~~~~~
	drivers/media/tuners/r820t.c:345:18: warning: 'r820t_lna_gain_steps' defined but not used [-Wunused-const-variable=]
	 static const int r820t_lna_gain_steps[]  = {
	                  ^~~~~~~~~~~~~~~~~~~~

They're actually used only by a routine that it is currently
commented out. So, move those tables to be together with such
code and comment them out.

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