[media] drxd_hard: get rid of warning: no previous prototype

drivers/media/dvb-frontends/drxd_hard.c:1751:5: warning: no previous prototype for 'SetOperationMode' [-Wmissing-prototypes]
drivers/media/dvb-frontends/drxd_hard.c:2615:5: warning: no previous prototype for 'DRXD_init' [-Wmissing-prototypes]
drivers/media/dvb-frontends/drxd_hard.c:2777:5: warning: no previous prototype for 'DRXD_status' [-Wmissing-prototypes]

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