usb: dwc2: move usb_disabled() call to host driver only

Since platform.c will get built for both Host and Gadget, if we leave the
usb_disabled() call in platform.c, it results in the following build error
when (!USB && USB_GADGET) condition is met.

ERROR: "usb_disabled" [drivers/usb/dwc2/dwc2_platform.ko] undefined!

Since usb_disabled() is mostly used to disable USB host functionality, move
the call the host portion for the DWC2 driver.

Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>