media: dvb: Fix section pipe handling

Calculation of pending data size in the section pipe, and other polled
pipes as well, added 1 extra byte which is correct only for the first
calculation.
In addition lookup of the feed object did not make sure feed type
is indeed a section feed, which could lead to undefined behavior.

Change-Id: Ic1699161106ab2183f8e08b02b78d9fe28b582f3
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>