usb : replace dma_pool_alloc and memset with dma_pool_zalloc

replace dma_pool_alloc and memset with a single call to dma_pool_zalloc

Signed-off-by: Saurabh Sengar <saurabh.truth@gmail.com>
Acked-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>