V4L/DVB (4292): Fix DISEQC regression

The changes to add frontend reinitialisation moved the position where the
init() op is called into the frontend thread. Unfortunately, since DISEQC
operations do not use the frontend thread, this meant that DISEQC could be
called against an uninitalised frontend, leading to all sorts of trouble.
Patch fixes this by reinstating the original fronted intialisation call.

Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>