staging: slicoss: fix use-after-free bug in slic_entry_remove

staging: slicoss: fix use-after-free bug in slic_entry_remove

This patch fixes a use-after-free bug that causes a null pointer
dereference in slic_entry_halt.

Since unregister_netdev() will ultimately call slic_entry_halt (the
net_device ndo_stop() virtual function for this device), we should
call it before freeing the memory used by slic_entry_halt.

Signed-off-by: David Matlack <dmatlack@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>