[media] exynos4-is: use monotonic timestamps as advertized

The exynos4 fimc capture driver claims to use monotonic
timestamps but calls ktime_get_real_ts(). This is both
an incorrect API use, and a bad idea because of the y2038
problem and the fact that the wall clock time is not reliable
for timestamps across suspend or settimeofday().

This changes the driver to use the normal v4l2_get_timestamp()
function like all other drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>