hostap: fix "hostap: proc: Use remove_proc_subtree()"

remove_proc_subtree() doesn't work here as local->ddev has already
been removed, and NULLed out.  Use proc_remove() instead.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>