hostap: proc: Use remove_proc_subtree()

Use remove_proc_subtree() rather than remove_proc_entry() to remove a
device-specific proc directory and all its children.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jouni Malinen <j@w1.fi>
cc: Johannes Berg <johannes@sipsolutions.net>
cc: linux-wireless@vger.kernel.org
cc: devel@driverdev.osuosl.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>