[media] radio-isa: get rid of warning: no previous prototype

drivers/media/radio/radio-isa.c:194:24: warning: no previous prototype for 'radio_isa_alloc' [-Wmissing-prototypes]
drivers/media/radio/radio-isa.c:210:5: warning: no previous prototype for 'radio_isa_common_probe' [-Wmissing-prototypes]
drivers/media/radio/radio-isa.c:290:5: warning: no previous prototype for 'radio_isa_common_remove' [-Wmissing-prototypes]

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