[media] ngene: Fix compilation when cxd2099 is not enabled

drivers/media/dvb/ngene/ngene-i2c.o: In function `cxd2099_attach':
ngene-i2c.c:(.text+0x195): multiple definition of `cxd2099_attach'
drivers/media/dvb/ngene/ngene-core.o:ngene-core.c:(.text+0x100): first defined here
drivers/media/dvb/ngene/ngene-cards.o: In function `cxd2099_attach':
ngene-cards.c:(.text+0x116): multiple definition of `cxd2099_attach'
drivers/media/dvb/ngene/ngene-core.o:ngene-core.c:(.text+0x100): first defined here
drivers/media/dvb/ngene/ngene-dvb.o: In function `cxd2099_attach':
ngene-dvb.c:(.text+0x7ad): multiple definition of `cxd2099_attach'
drivers/media/dvb/ngene/ngene-core.o:ngene-core.c:(.text+0x100): first defined here

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