usbatm: fix tiny race

If usbatm_do_heavy_init finishes before usbatm_heavy_init
writes the pid, the disconnect method could shoot down the
wrong process if the pid has been recycled.

Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>