USB_DWCOTG: Disable building dwc_otg as a module (raspberrypi#2265)

When dwc_otg is built as a module, build will fail with the following
error:

ERROR: "DWC_TASK_HI_SCHEDULE" [drivers/usb/host/dwc_otg/dwc_otg.ko] undefined!
scripts/Makefile.modpost:91: recipe for target '__modpost' failed
make[1]: *** [__modpost] Error 1
Makefile:1199: recipe for target 'modules' failed
make: *** [modules] Error 2

Even if the error is solved by including the missing
DWC_TASK_HI_SCHEDULE function, the kernel will panic when loading
dwc_otg.

As a workaround, simply prevent user from building dwc_otg as a module
as the current kernel does not support it.

See: raspberrypi#2258

Signed-off-by: Malik Olivier Boussejra <[email protected]>

drivers/usb/host/Kconfig

@@ -772,7 +772,7 @@ config USB_HWA_HCD
 	  will be called "hwa-hc".
 
 config USB_DWCOTG
-	tristate "Synopsis DWC host support"
+	bool "Synopsis DWC host support"
 	depends on USB && (FIQ || ARM64)
 	help
 	  The Synopsis DWC controller is a dual-role
@@ -781,9 +781,6 @@ config USB_DWCOTG
 	  Enable this option to support this IP in host controller mode.
 	  If unsure, say N.
 
-	  To compile this driver as a module, choose M here: the
-	  modules built will be called dwc_otg and dwc_common_port.
-
 config USB_IMX21_HCD
        tristate "i.MX21 HCD support"
        depends on ARM && ARCH_MXC