[media] s5p-mfc: Fix selective sclk_mfc init

fc906b6d "Remove special clock usage in driver" removed
initialization of MFC special clock, arguing that there's
no need to do it explicitly, since it's one of MFC gate clock's
dependencies and gets enabled along with it. However, there's
no promise of keeping this hierarchy across Exynos SoC
releases, therefore this approach fails to provide a stable,
portable solution.

Out of all MFC versions, only v6 doesn't use special clock at all.
For other versions log a message only in case clk_get fails,
as not all the devices with the same MFC version require
initializing the clock explicitly.

Signed-off-by: Mateusz Zalega <m.zalega@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>