Update vme4l-core.c

If the vme4l_core issues a DMA-Transfer to userspace a general protection faultarises due to a wrong flag setup in get_user_pages_fast().

With: rv = get_user_pages_fast( uaddr, nr_pages, direction == DMA_FROM_DEVICE ? FOLL_WRITE : 0, pages);

General protection fault doesn't ocurr