usb gadget: link fixes for MIDI gadget

Change how the MIDI gadget driver builds:  don't use separate
compilation, since it works poorly when key parts are library
code (with init sections etc).  Instead be as close as we can
to "gcc --combine ...".

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