USB: dma: remove unused function prototype

usb_buffer_map_sg() and usb_buffer_unmap_sg() have no definition
since the beginning of v5.4. The rest are gone from 2.6.12.

Signed-off-by: Randy Li <ayaka@soulik.info>
Link: https://lore.kernel.org/r/20230914172336.18761-2-ayaka@soulik.info


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>