usb: Fix bug when both DFU & ETHER are defined

When both CONFIG_USB_GADGET & CONFIG_USB_ETHER are defined
the makefile links objects twice.

This patch uses a Makefile specific idiom of
'if defined(CONFIG_USB_GADGET) || defined(CONFIG_USB_ETHER)'
to handle the case.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>