greybus: es1: fix DMA-buffer on stack

A stack-allocated buffer is not generally DMA-able and must not be used
for USB control transfers.

Note that the memset and extra buffer byte were redundant as no more
than the bytes actually transferred was ever added to the fifo.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>