wusb: Stop using the stack for sg crypto scratch space

Pointing an sg list at the stack is verboten and, with
CONFIG_VMAP_STACK=y, will malfunction.  Use kmalloc for the wusb
crypto stack space instead.

Untested -- I'm not entirely convinced that this hardware exists in
the wild.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>