[media] tc90522: declare tc90522_functionality as static

drivers/media/dvb-frontends/tc90522.c:706:5: warning: symbol 'tc90522_functionality' was not declared. Should it be static?
drivers/media/dvb-frontends/tc90522.c:706:5: warning: no previous prototype for 'tc90522_functionality' [-Wmissing-prototypes]
 u32 tc90522_functionality(struct i2c_adapter *adap)
     ^

Cc: Akihiro Tsukada <tskd08@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>