vme_user: Add ioctl for destroying windows

The new ioctl VME_DESTROY_WINDOW can be used on
master or slave window device files.

Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>
vme: Propagate vme_irq_free failure reasons upwards

The function can fail for various reasons, and callers
may want to learn what exactly went wrong, respectively
that the operation failed at all.

Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>
vme_user: add ioctls for registering and deregistering irqs

Currently, the irqs are only registered with and removed from
the generic VME layer, but the results is not yet propagated
into userspace.

TODO: Use UIO and the respective audis mechanism to bring the irq
upwards.

Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>
vme_user: Make sysfs names more sensible

Specifying a path name in device_create does not
automatically generate the path, so change the
sysfs file names from bus/vme/mX (which is mapped
to bus!vme!mX) to b0mX, and similar for the slave
window and control files.

Currently, the driver is limited to one single bus,
but the name format allows for easily extending the
scheme to multiple buses.

Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>
left-overs that need to be put into commits