media: dvb_demux: Better handle discontinuity errors

When a packet discontinuity happens, it is not just the payload
that was lost. The headers are lost too. So, the max size is not
184 but, instead 188.

Also, while printing warnings, make a distinction between
MPEG-TS indicated discontinuity and detected one.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>