dmaengine: pch_dma: 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 pch dma driver we need to use free_irq() before tasklet_kill(). So move
up the free_irq() which will ensure that the irq is disabled and also wait till
all scheduled interrupts are executed by invoking synchronize_irq().

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

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>