media: uvcvideo: Use ktime_t for stats

'struct timespec' works fine here, but we try to migrate
away from it in favor of ktime_t or timespec64. In this
case, using ktime_t produces the simplest code.

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>