i2c: exynos5: simplify timings calculation

Instead of using cryptic loop direct calculation of timings
can be used.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>