usb: host: ehci: Use dma_pool_zalloc()

Use dma_pool_zalloc() instead of dma_pool_alloc + memset

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>