[media] saa7164-core: declare symbols as static

Those symbols are used only at saa7164-core.

drivers/media/pci/saa7164/saa7164-core.c:55:14: warning: symbol 'fw_debug' was not declared. Should it be static?
drivers/media/pci/saa7164/saa7164-core.c:75:14: warning: symbol 'print_histogram' was not declared. Should it be static?
drivers/media/pci/saa7164/saa7164-core.c:83:14: warning: symbol 'guard_checking' was not declared. Should it be static?

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