pinctrl: mcp23s08: debugfs: remove custom printer

The comment for this dbg_show says that it is supposed to return more
than what the generic code is showing, including de-glitching. That's
wrong because:

- this chip does not support deglitching,
- the code does not print anything extra compared to the generic
handler,
- its behavior is different because it skips unrequested GPIOs; the
generic code prints their names if they're assigned

There is an important difference, though. Previously, dbg_show would
re-check some registers to see if they still match what the regmap
thinks should be in there. This was semi-useful when develpoing the HW
board because it immediately pointed to SPI wiring problem if a CS
connection was missing (0xffs are easy to see). However, I do not think
that this makes much sense -- and one could always do this in some other
way if needed.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Cc: Phil Reid <preid@electromag.com.au>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>