offb: Fix bug in calculating requested vram size

commit c055fe07 upstream.

We used to try to request 8 times more vram than needed, which would
fail if the card has a too small BAR (observed with qemu & kvm).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>