usb: dual-role: make stub functions inline

If CONFIG_DUAL_ROLE_USB_INTF is disabled but the exported functions
are referenced, the following build warning results:

	In file included from include/linux/usb/class-dual-role.h:112:13:
	warning: ‘dual_role_instance_changed’ defined but not used
	[-Wunused-function]

These stub functions should be static inline.

Change-Id: I5a9ef58dca32306fac5a4c7f28cdaa36fa8ae078
Signed-off-by: Jack Pham <jackp@codeaurora.org>