spi: spi-fsl-dspi: Exit the ISR with IRQ_NONE when it's not ours

The DSPI interrupt can be shared between two controllers at least on the
LX2160A. In that case, the driver for one controller might misbehave and
consume the other's interrupt. Fix this by actually checking if any of
the bits in the status register have been asserted.

Fixes: 13aed23927 ("spi: spi-fsl-dspi: use IRQF_SHARED mode to request IRQ")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20190822212450.21420-2-olteanv@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org

drivers/spi/spi-fsl-dspi.c

@@ -886,9 +886,11 @@ static irqreturn_t dspi_interrupt(int irq, void *dev_id)
 					trans_mode);
 			}
 		}
+
+		return IRQ_HANDLED;
 	}
 
-	return IRQ_HANDLED;
+	return IRQ_NONE;
 }
 
 static const struct of_device_id fsl_dspi_dt_ids[] = {