[media] ttpci: cleanup a bogus smatch warning

Cleanup this bogus smatch warning:
	drivers/media/pci/ttpci/budget.c:635 frontend_init() warn: missing break? reassigning 'budget->dvb_frontend'

And document the fall through logic at the switch().

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