mfd: sm501 build fixes when CONFIG_MFD_SM501_GPIO unset

Fix the build problems if CONFIG_MFD_SM501_GPIO is not set, which is
generally when there is no gpiolib support available as currently happens
on x86 when building PCI SM501.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Tested-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>