[PATCH] spi: bcm2835aux: ensure interrupts are enabled for shared handler

Rob Herring [off-list ref] writes:

The BCM2835 AUX SPI has a shared interrupt line (with AUX UART).
Downstream fixes this with an AUX irqchip to demux the IRQ sources and a
DT change which breaks compatibility with older kernels. The AUX irqchip
was already rejected for upstream[1] and the DT change would break
working systems if the DTB is updated to a newer one. The latter issue
was brought to my attention by Alex Graf.

The root cause however is a bug in the shared handler. Shared handlers
must check that interrupts are actually enabled before servicing the
interrupt. Add a check that the TXEMPTY or IDLE interrupts are enabled.

It looks to me like we'd only return IRQ_HANDLED if we did work that
needed doing.  Is this check effectively doing some interlock to make
sure that we've already started bcm2835aux_spi_transfer_one_irq() and
aren't just racing against transaction setup?
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