pinctrl: aspeed: Fix confusing types in return value

The function signature is int, but we return a bool. Instead return a
negative errno as the kerneldoc suggests.

Fixes: 4d3d0e42

 ("pinctrl: Add core support for Aspeed SoCs")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20230119231856.52014-1-joel@jms.id.au
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>