USB: make usb_kbd obey USB DMA alignment requirements

Change usb_kbd driver to obey alignment requirements for USB DMA on
the buffer used for data transfer.  This is necessary for
architectures that enable dcache and enable USB DMA.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>