staging: panel: fix stackdump

if we load the module, unload and then again try to load the module, we
will get a stackdump. In the module_exit function we are unregistering
the device and releasing the parport. So when we reach the detach
function parport is already null and the unregister_reboot_notifier()
is never called. When we again try to load the module it again tries
register_reboot_notifier() and gives us a stackdump as its earlier
registration is still not removed. It was caused by the
commit bb046fef ('staging: panel: register reboot')
Fix this by moving all the unregistering and releasing in the detach
function, which should be the ideal case as the detach will be called if
we try to unregister the driver or if the parport is removed.

Fixes: bb046fef

 ('staging: panel: register reboot')
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>