usb: diag: Replace kref_put_spinlock_irqsave() with kref_put_lock()

The kref structure has been changed to use refcount_t instead of
atomic_t. But also a new API function kref_put_lock() was added.
We can use this in place of defining our own locking version of
kref_put().

Change-Id: I0f7dc0c7c24f2b8e577b3b67cdd9422c4bd35514
Signed-off-by: Jack Pham <jackp@codeaurora.org>