usb: dwc3: fix compile failure if config host only mode

usb: dwc3: fix compile failure if config host only mode

This patch fixes following compile error in dwc3 if select
CONFIG_USB_DWC3_HOST.

drivers/usb/dwc3/core.c:874: undefined reference to `dwc3_gadget_restart'
drivers/usb/dwc3/core.c:880: undefined reference to `dwc3_gadget_restart'

which was caused by commit
commit 9607f47dfec23c5773d74e45ed561859eabce2b7
usb: dwc3: add functions to set force mode

Change-Id: Id0abaf89fba006609dbf2e7a771149453465b371
Signed-off-by: Wu Liang feng <wulf@rock-chips.com>