[media] radio-sf16fmr2: declare some structs as static

drivers/media/radio/radio-sf16fmr2.c:308:19: warning: symbol 'fmr2_isa_driver' was not declared. Should it be static?
drivers/media/radio/radio-sf16fmr2.c:316:19: warning: symbol 'fmr2_pnp_driver' was not declared. Should it be static?

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