[media] c8sectpfe: Fix broken circular buffer wp management

During the review process, a regression was intoduced in the
circular buffer write pointer management. This means that wp
doesn't get managed properly once the buffer becomes full.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>