mfd: sgi-ioc3: Turn Kconfig option into a bool

Module builds of ioc3 fail with following errors:

ERROR: "spurious_interrupt" [drivers/mfd/ioc3.ko] undefined!
ERROR: "pci_find_host_bridge" [drivers/mfd/ioc3.ko] undefined!

Exporting pci_find_host_bridge got rejected by maintainer, so easiest
fix is to disable module building, which even makes sense since both
SGI Origin and Octane always contain at least one IOC3 chip.

Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/Kconfig

···
 	  device found on several devices in RAVE line of hardware.
 
 config SGI_MFD_IOC3
-	tristate "SGI IOC3 core driver"
+	bool "SGI IOC3 core driver"
 	depends on PCI && MIPS && 64BIT
 	select MFD_CORE
 	help