V4L/DVB (9928): Convert GP8PSK module to use S2API

This patch converts the gp8psk module to use the S2API.
It pretends to be  DVB-S2 capable in order to allow the various
supported modulations (8PSK, QPSK-Turbo, etc), and keep software
compatibility with the S2API patches for Mythtv and VDR.

Signed-off by: Alan Nisota <alannisota@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>