i2c: isch: Add module parameter for backbone clock rate if divider is unset

It was observed the Host Clock Divider was not written by the driver. It
was still set to (default) 0, if not already set by BIOS, which caused
garbage on SMBus.
This driver adds a parameters which is used to calculate the divider
appropriately for a default bitrate of 100 KHz. This new divider is only
applied if the clock divider is still default 0.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Reviewed-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Wolfram Sang <wolfram@the-dreams.de>