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

USB_DWCOTG: Disable building dwc_otg as a module (#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: https://github.com/raspberrypi/linux/issues/2258

Signed-off-by: Malik Olivier Boussejra <malik@boussejra.com>