staging: unisys: visornic: handle error return from device registration

There is no code to handle an error return in visornic, when it tries to
register with visorbus. This patch handles an error return from
visorbus_register_visor_driver() by dropping out of initialization.

Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>