airo: Use remove_proc_subtree()

Use remove_proc_subtree() to remove the airo device subdir and all its
children instead of doing it manually.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-wireless@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>