[media] dvb-usb: hide unused functions

A couple of data structures in the dibusb-common file are only
accessed when CONFIG_DVB_DIB3000MC is enabled, otherwise we
get a harmless gcc warning:

usb/dvb-usb/dibusb-common.c:223:34: error: 'dib3000p_panasonic_agc_config' defined but not used
usb/dvb-usb/dibusb-common.c:211:32: error: 'stk3000p_dib3000p_config' defined but not used

This moves the existing #ifdef a few lines up to correctly cover
all the conditional data structures, which gets rid of the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>