[media] au0828: fix i2c clock speed for DViCO FusionHDTV7

DViCO FusionHDTV7 device that use au0828 can fail to communicate with
xc5000 using i2c interface because of high i2c clock speed - i2c clock
stretching bug. It causes to fail xc5000 firmware loading normally at
the current driver.

Already this problem fixed as changing to low i2c clock speed at
HVR-950q device, also DViCO FusionHDTV7 device can solve it as using low
i2c clock speed - 20KHz.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>