USB: usbatm: convert heavy init dances to kthread API

This is an attempt to kill two birds with one stone.

First, we kill one more user of kernel_thread, which is scheduled
for removal. Second - we kill one of the last users of kill_proc -
the function which is also to be removed, because it uses a pid_t
which is not safe now.

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