V4L/DVB (7197): bttv: Fix overlay divide error

The initial work to convert the bttv driver to V4L2 "Partial conversion
from V4L1 to V4L2" (e84619b1), missed
the line which set the appropriate overlay crop structure in the newly
allocated bttv_buffer.  This then causes a divide error in the
bttv_calc_geo function.

Signed-off-by: Robert Fitzsimons <robfitz@273k.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>