gpio: aspeed: fix compile testing warning

Gcc cannot always see that BUG_ON(1) is guaranteed to not
return, so we get a warning message in some configurations:

drivers/gpio/gpio-aspeed.c: In function 'bank_reg':
drivers/gpio/gpio-aspeed.c:244:1: error: control reaches end of non-void function [-Werror=return-type]

Using a plain BUG() is easier here and avoids the problem.

Fixes: 44ddf559 ("gpio: aspeed: Rework register type accessors")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>