[PATCH] USB: fix drivers/usb/gadget/ether.c compile error

This fixes a compile glitch with CONFIG_USB_ETH_RNDIS disabled, and
replaces some inline #ifdeffery (and other code) with inline functions
which can evaluate to constants.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>