[media] rtl2830: declare functions as static

drivers/media/dvb-frontends/rtl2830.c:21:5: warning: no previous prototype for ‘rtl2830_bulk_write’ [-Wmissing-prototypes]
 int rtl2830_bulk_write(struct i2c_client *client, unsigned int reg,
     ^
drivers/media/dvb-frontends/rtl2830.c:33:5: warning: no previous prototype for ‘rtl2830_update_bits’ [-Wmissing-prototypes]
 int rtl2830_update_bits(struct i2c_client *client, unsigned int reg,
     ^
drivers/media/dvb-frontends/rtl2830.c:45:5: warning: no previous prototype for ‘rtl2830_bulk_read’ [-Wmissing-prototypes]
 int rtl2830_bulk_read(struct i2c_client *client, unsigned int reg, void *val,
     ^

Cc: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>