spi: xilinx: Return IRQ_NONE if no interrupts were detected

Return IRQ_NONE from the interrupt handler if the handler is running, but
no interrupt was detected. This allows the system to recover in case of an
interrupt storm due to an invalid interrupt configuration or faulty
hardware.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>