HID: fix possible deadlock in usbhid_close()

This patch switches usbhid_close() from flush_scheduled_work() to canceling
the outstanding work. This fixes a possible deadlock due to work taking
the mutex usbhid_close() holds. Lockdep reported the problem.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
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Signed-off-by: Jiri Kosina <jkosina@suse.cz>