V4L/DVB (6343): ivtvfb: check return value of unregister_framebuffer

Prevent unloading the framebuffer if it is still in use.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>