spi: spi-fsl-dspi: Fix interrupt-less DMA mode taking an XSPI code path

[ Upstream commit 826b3a6a34619b934cdc33eeb961fcb99ce92c09 ]

Interrupts are not necessary for DMA functionality, since the completion
event is provided by the DMA driver.

But if the driver fails to request the IRQ defined in the device tree,
it will call dspi_poll which would make the driver hang waiting for data
to become available in the RX FIFO.

Fixes: c55be305 ("spi: spi-fsl-dspi: Use poll mode in case the platform IRQ is missing")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20200318001603.9650-9-olteanv@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>
Stable-dep-of: fa60c094c19b ("spi: spi-fsl-dspi: Clear completion counter before initiating transfer")
Signed-off-by: Sasha Levin <sashal@kernel.org>