The logic at em28xx_isoc_dvb_max_packetsize() sucks, at least for newer
the needed packet size. Yet, it is better than nothing.

Rewrite the code in order to change the default to 752 for em2884 and
newer chips and provide a better way to handle per-chipset specifics.

For em2874, the current default should be enough, as the only em2874
board is currently a 1-seg ISDB-T board, so, it needs only a limited
amount of bandwidth.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>