gpio: aspeed: avoid return type warning

gcc has a hard time tracking whether BUG_ON(1) ends
execution or not:

drivers/gpio/gpio-aspeed-sgpio.c: In function 'bank_reg':
drivers/gpio/gpio-aspeed-sgpio.c:112:1: error: control reaches end of non-void function [-Werror=return-type]

Use the simpler BUG() that gcc knows cannot continue.

Fixes: f8b410e3 ("gpio: aspeed-sgpio: Rename and add Kconfig/Makefile")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>