[media] au0828: improve I2C speed

[media] au0828: improve I2C speed

Commits 21dc61d3c0a4 and 7a1dd50b89d4 reduced the board I2C
speed to 20 MHz by default, due to a I2C stretch issue:
while xc5000 uses i2c stretch when a command is sent to it,
au0828 doesn't support this feature.

However, this is needed only for Xceive tuners. The other
I2C devices can work at the max speed.

So, revert the workarounds at board level, handling it at
I2C level, only when talking with xc5000.

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