[media] rtl2832: declare functions as static

drivers/media/dvb-frontends/rtl2832.c:157:5: warning: no previous prototype for ‘rtl2832_bulk_write’ [-Wmissing-prototypes]
 int rtl2832_bulk_write(struct i2c_client *client, unsigned int reg,
     ^
drivers/media/dvb-frontends/rtl2832.c:169:5: warning: no previous prototype for ‘rtl2832_update_bits’ [-Wmissing-prototypes]
 int rtl2832_update_bits(struct i2c_client *client, unsigned int reg,
     ^
drivers/media/dvb-frontends/rtl2832.c:181:5: warning: no previous prototype for ‘rtl2832_bulk_read’ [-Wmissing-prototypes]
 int rtl2832_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>