i2c-davinci: Ensure clock between 7-12 MHz

Ensure psc value gives a clock between 7-12 MHz

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>