media: uvcvideo: Use ktime_t for timestamps

uvc_video_get_ts() returns a 'struct timespec', but all its users
really want a nanoseconds variable anyway.

Changing the deprecated ktime_get_ts/ktime_get_real_ts to ktime_get
and ktime_get_real simplifies the code noticeably, while keeping
the resulting numbers unchanged.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>