media: af9035: add the formula used for the I2C speed

A very old patch sent to the media ML used to contain the
I2C speed formula:

	https://lore.kernel.org/linux-media/1312539895.2763.33.camel@Jason-Linux/



When the ite9135 code was merged with af9035, the formula was
lost. As we might need to slow down the speed for some devices,
add the formula again.

No functional changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>