USB: ene_usb6250: fix DMA to the stack

The ene_usb6250 sub-driver in usb-storage does USB I/O to buffers on
the stack, which doesn't work with vmapped stacks.  This patch fixes
the problem by allocating a separate 512-byte buffer at probe time and
using it for all of the offending I/O operations.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-and-tested-by: Andreas Hartmann <andihartmann@01019freenet.de>
CC: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>