dmaengine: mediatek: Fix deadlock caused by synchronize_irq()

The synchronize_irq(c->irq) will not return until the IRQ handler
mtk_uart_apdma_irq_handler() is completed. If the synchronize_irq()
holds a spin_lock and waits the IRQ handler to complete, but the
IRQ handler also needs the same spin_lock. The deadlock will happen.
The process is shown below:

cpu0 cpu1
mtk_uart_apdma_device_pause() | mtk_uart_apdma_irq_handler()
spin_lock_irqsave() |
| spin_lock_irqsave()
//hold the lock to wait |
synchronize_irq() |

This patch reorders the synchronize_irq(c->irq) outside the spin_lock
in order to mitigate the bug.

Fixes: 9135408c3ace ("dmaengine: mediatek: Add MediaTek UART APDMA support")
Signed-off-by: Duoming Zhou <[e-mail]>
Reviewed-by: Eugen Hristev <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/mediatek/mtk-uart-apdma.c

···
 	mtk_uart_apdma_write(c, VFF_EN, VFF_EN_CLR_B);
 	mtk_uart_apdma_write(c, VFF_INT_EN, VFF_INT_EN_CLR_B);
 
-	synchronize_irq(c->irq);
-
 	spin_unlock_irqrestore(&c->vc.lock, flags);
+	synchronize_irq(c->irq);
 
 	return 0;
 }