[media] stb0899: store successful inversion for next run

Usually, inversion doesn't change in a system. Storing the last
successful inversion value speeds up tuning of DVB-S2 transponders.

Signed-off-by: Reinhard Nißl <rnissl@gmx.de>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>