V4L/DVB (6006): tuner: move last_div to tuner-simple private data

tuner-simple is the only sub-driver that uses last_div, so we 
can free up two bytes of memory for all other tuners, by moving 
this into tuner-simple's private data area.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>