media: vivid: fix gain when autogain is on

In the vivid driver you want gain to continuous change while autogain
is on. However, dev->jiffies_vid_cap doesn't actually change. It probably
did in the past, but changes in the code caused this to be a fixed value
that is only set when you start streaming.

Replace it by jiffies, which is always changing.

[mchehab@kernel.org: use jiffies_to_msecs() instead of dividing by HZ]

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>