i2c: davinci: Avoid zero value of CLKH

If CLKH is set to 0 I2C clock is not generated at all, so avoid this value
and stretch the clock in this case.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>