media: uvcvideo: Fixed ktime_t to ns conversion

Commit 828ee8c7 ("media: uvcvideo: Use ktime_t for timestamps")
changed to use ktime_t for timestamps. Older Kernels use a struct for
ktime_t, which requires the conversion function ktime_to_ns to be used on
some places. With this patch it will compile now also for older Kernel
versions.

Signed-off-by: Jasmin Jessich <jasmin@anw.at>
Acked-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>