usb gadget stack: remove usb_ep_*_buffer(), part 1

Remove usb_ep_{alloc,free}_buffer() calls, for small dma-coherent buffers.
This patch just removes the interface and its users; later patches will
remove controller driver support.

  - This interface is invariably not implemented correctly in the
    controller drivers (e.g. using dma pools, a mechanism which
    post-dates the interface by several years).

  - At this point no gadget driver really *needs* to use it.  In
    current kernels, any driver that needs such a mechanism could
    allocate a dma pool themselves.

Removing this interface is thus a simplification and improvement.

Note that the gmidi.c driver had a bug in this area; fixed.

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