V4L/DVB (13048): dib0070: fix build dependency when driver is disabled

Fix build dependency on function, dib0070_ctrl_agc_filter

[mchehab@redhat.com: reverted the quick workaround 3f48258e]

Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Patrick Boettcher <pboettcher@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>