Apart from the usual stuff, the debugfs code is currently also used for
one non-obvious side effect. It attempts to check whether the chip's
registers are still set to an expected value, and if not, re-initializes
them. It seems that the driver has "always" done so.

The code, however, also checks the INTF register which normally
indicates which pins have caused the recent interrupt. That's a volatile
register, and the datasheet says that writes are ignored in there.

When I'm just cat-ing /sys/kernel/debug/gpio with no SPI traffic and no
nosie on the GPIO lines, I'm not getting any warnings. Once I actually
use these GPIOs and cat that file in parallel, I always seem to get a:

  mcp23s08 spi1.1: restoring reg 0x07 from 0x0000 to 0xffff (power-loss?)

This might be a sign that I should not leave my unused inputs floating,
but the code should not be checking a volatile register, anyway. Let's
simply skip this last item in the iteration. I was also considering
removing this enitre re-initialization because it's non-obvious, but the
code survived various refactorings already and has sign-offs by people
who know more than I do, so let's leave it as-is. For now :).
Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Reviewed-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>