media: dvb-core: Fix receiving invalid EIT-sections

Resetting buf without resetting pusi_seen at a channel-switch can lead
to copying the rest of a section to the start of buf, but treating it as
a complete section, when the next pusi arrives.
EIT-sections starting without valid header were randomly received during
an EIT-scan on a transponder.

Signed-off-by: Johann Friedrichs <johann.friedrichs@web.de>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>