Exynos: Clock: Fix exynos5_get_periph_rate for I2C.

Commit 2e82e925 'Exynos: Clock: Cleanup
soc_get_periph_rate' introduced a bug in I2C config. This patch makes cros_ec
keyboard working again on Samsung Chromebook (snow).

Changes in V2: reorder lines as requested by Joonyoung Shim.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>
Cc: Akshay Saraswat <akshay.s@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Reviewed-by: Simon Glass <sjg@chroimum.org>
Tested-by: Simon Glass <sjg@chroimum.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>