staging: vme_pio2: fix oops on module unloading

This patch forbids loading vme_pio2 module without specifing "num_bus" parameter.
Otherwise on module unloading pio2_exit() calls vme_unregister_driver() for not
registered pio2_driver.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Manohar Vanga <manohar.vanga@gmail.com>
Acked-by: Martyn Welch <martyn.welch@ge.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>