i2c: exynos5: de-duplicate error logs on clock setup

In case of clock setup error it is enough to log it once.
Moreover patch simplifies clock setup routines.

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