spi: spi-geni-qcom: Fix geni_spi_isr() NULL dereference in timeout case

In commit 7ba9bdcb ("spi: spi-geni-qcom: Don't keep a local state
variable") we changed handle_fifo_timeout() so that we set
"mas->cur_xfer" to NULL to make absolutely sure that we don't mess
with the buffers from the previous transfer in the timeout case.

Unfortunately, this caused the IRQ handler to dereference NULL in some
cases.  One case:

  CPU0                           CPU1
  ----                           ----
                                 setup_fifo_xfer()
                                  geni_se_setup_m_cmd()
                                 <hardware starts transfer>
                                 <transfer completes in hardware>
                                 <hardware sets M_RX_FIFO_WATERMARK_EN in m_irq>
                                 ...
                                 handle_fifo_timeout()
                                  spin_lock_irq(mas->lock)
                                  mas->cur_xfer = NULL
                                  geni_se_cancel_m_cmd()
                                  spin_unlock_irq(mas->lock)

  geni_spi_isr()
   spin_lock(mas->lock)
   if (m_irq & M_RX_FIFO_WATERMARK_EN)
    geni_spi_handle_rx()
     mas->cur_xfer NULL dereference!

tl;dr: Seriously delayed interrupts for RX/TX can lead to timeout
handling setting mas->cur_xfer to NULL.

Let's check for the NULL transfer in the TX and RX cases and reset the
watermark or clear out the fifo respectively to put the hardware back
into a sane state.

NOTE: things still could get confused if we get timeouts all the way
through handle_fifo_timeout() and then start a new transfer because
interrupts from the old transfer / cancel / abort could still be
pending.  A future patch will help this corner case.

Fixes: 561de45f

 ("spi: spi-geni-qcom: Add SPI driver support for GENI based QUP")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20201217142842.v3.1.I99ee04f0cb823415df59bd4f550d6ff5756e43d6@changeid
Signed-off-by: Mark Brown <broonie@kernel.org>