i2c-bcm2708: fixed baudrate

Fixed issue where the wrong CDIV value was set for baudrates below 3815 Hz (for 250MHz bus clock). In that case the computed CDIV value was more than 0xffff. However the CDIV register width is only 16 bits. This resulted in incorrect setting of CDIV and higher baudrate than intended. 
Example: 3500Hz -> CDIV=0x11704 -> CDIV(16bit)=0x1704 -> 42430Hz