i2c: i2c-omap: Fix standard and fast mode prescalers

The prescalers for 100 kHz and 400 kHz mode
are wrong for omap 3430 and omap 2430. The
internal clock is the fclock divided by the
prescaler. The PSC is an 8 bit field in
omap3430 and omap2430. Moreover, the scll and
sclh values should be adjusted properly.
Having the correct prescaler is important in
the process of getting a finite i2c clock. In
addition, the prescaler is used in the process
of activating the correct noise filter and thus,
lets more error resilient i2c communications.

Signed-off-by: Eero Nurkkala <ext-eero.nurkkala@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>