dmaengine: at_hdmac: use tasklet_kill in teardown

dmaengine: at_hdmac: use tasklet_kill in teardown

As discussed in [1] the tasklet_disable is not a proper function for teardown.
We need to ensure irq is disabled, followed by ensuring that don't schedule any
more tasklets and then its safe to use tasklet_kill().

Here in at_hdmac driver we use free_irq() before tasklet_kill(). The free_irq()
will ensure that the irq is disabled and also wait till all scheduled interrupts
are executed by invoking synchronize_irq(). So we need to only do tasklet_kill()
after invoking free_irq()

[1]: http://lwn.net/Articles/588457/

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>