This patch fixes BUG: sleeping function called from invalid context in
stm32_dma_disable_chan function.

The goal of this function is to force channel disable if it has not been
disabled by hardware. This consists in clearing STM32_DMA_SCR_EN bit and
read it as 0 to ensure the channel is well disabled and the last transfer
is over.

In previous implementation, the waiting loop was based on a do...while (1)
with a call to cond_resched to give the scheduler a chance to run a higher
priority process.

But in some conditions, stm32_dma_disable_chan can be called while
preemption is disabled, on a stm32_dma_stop call for example. So
cond_resched must not be used.

To avoid this, use readl_relaxed_poll_timeout_atomic to poll
STM32_DMA_SCR_EN bit cleared.
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20200129153628.29329-8-amelie.delaunay@st.comSigned-off-by: Vinod Koul <vkoul@kernel.org>