USB: serial: iuu_phoenix: fix DMA from stack

commit 54d0a3ab upstream.

Stack-allocated buffers cannot be used for DMA (on all architectures) so
allocate the flush command buffer using kmalloc().

Fixes: 60a8fc01

 ("USB: add iuu_phoenix driver")
Cc: stable <stable@vger.kernel.org>     # 2.6.25
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>