fbdev: uvesafb: Call cn_del_callback() at the end of uvesafb_exit()

Delete the v86d netlink only after all the VBE tasks have been
completed.

Fixes initial state restore on module unload:
uvesafb: VBE state restore call failed (eax=0x4f04, err=-19)

Signed-off-by: Jorge Maidana <jorgem.linux@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>