i2c-pnx: Limit maximum divider to 1023

Limit maximum divider to 0x3ff to divider computations. On high I2C
parent clock rates, the divider can exceed 0x3ff. This will help
prevent some very odd clock rates.

Signed-off-by: Kevin Wells <wellsk40@gmail.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>