[media] nuvoton-cir: get rid of warning: no previous prototype

drivers/media/rc/nuvoton-cir.c:1223:5: warning: no previous prototype for 'nvt_init' [-Wmissing-prototypes]
drivers/media/rc/nuvoton-cir.c:1228:6: warning: no previous prototype for 'nvt_exit' [-Wmissing-prototypes]

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