Bluetooth: hidp: Fix module reference cleanup

Calling module_put(THIS_MODULE) is *never* safe when we cannot go sure that we
own at least two references. This is because the call may unload our module
before it returns and then the "return" will jump into invalid memory.

Gladly, module.h provides a wrapper for kthread-users: module_put_and_exit().
This puts our module and then exits the kthread without returning to the module.

This patch fixes the hidp kthread to use this wrapper instead of manually
freeing its own reference. See nfsd or lockd for other kthreads using this.

Calling __module_get() inside the kthread is safe as the hidp module will always
wait until the kthread sets "waiting_for_startup" to 0.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>