net/usb/r815x: replace USB buffer from stack to DMA-able

Some USB buffers use stack which may not be DMA-able.
Use the buffers from kmalloc to replace those one.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>