usb: phy: allow modular build of DT helper

This one is a curious Kbuild glitch: The drivers/usb/phy/of.o file is currently always built-in if CONFIG_OF is set, independent of whether USB is modular or not.

For the case of CONFIG_USB=m && CONFIG_USB_GADGET=m && CONFIG_USB_PHY=n, however the file is not built at all, because drivers/Makefile only looks in drivers/usb/ for modules and not for built-in code. This results in a link error with in the chipidea module:

ERROR: of_usb_get_phy_mode [drivers/usb/chipidea/ci_hdrc.ko] undefined!

To work around this behavior, this changes the usb-phy Makefile to create a module named usb-phy-of.ko for this one file whenever both CONFIG_USB and CONFIG_USB_GADGET are set to 'm'. This seems to be the correct behavior as it also avoids unnecessary built-in code when everything else related to USB is a module, and it nicely avoids the link error.

index 19c0dccbb116..4367fbc1870b 100644
@@ -2,7 +2,11 @@
 # Makefile for physical layer USB drivers
 #
 obj-$(CONFIG_USB_PHY)			+= phy.o
-obj-$(CONFIG_OF)			+= of.o
+
+ifdef CONFIG_OF
+usb-phy-of-objs				:= of.o
+obj-$(CONFIG_USB_COMMON)		+= usb-phy-of.o
+endif
 
 # transceiver drivers, keep the list sorted