[media] redrat3: cap duration in the right place

Trying to cap duration before multiplying it was obviously wrong.

CC: Chris Dodge <chris@redrat.co.uk>
CC: Andrew Vincer <andrew.vincer@redrat.co.uk>
CC: Stephen Cox <scox_nz@yahoo.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>