V4L/DVB (8249): Fix pointer cast warnings in the ivtv framebuffer driver

Fix casts of pointers to integers of different sizes in the ivtv framebuffer
driver.

These were introduced in patch 38eb6ba5b8487d15b020fe391d324edd2b659b03.

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