[media] r820t: Remove a warning for an unused value

Currently, the driver complains about the pre_detect var:

	drivers/media/tuners/r820t.c: In function 'r820t_sysfreq_sel':
	drivers/media/tuners/r820t.c:722:31: warning: variable 'pre_dect' set but not used [-Wunused-but-set-variable]

While rtl8232 code comments it, perhaps some other driver may use.
So, the better is to keep the code there, allowing to enable it
via r820t config data.

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